Photo of Danny Berger
Search by Color with Elasticsearch
Post |

A year ago when I updated the TLE website I dropped the "search by color" functionality. Originally, all the colors were indexed into a database table and the frontend generated some complex queries to support specific and multi-color searching. On occasion, it caused some database bottlenecks during peak loads and with some particularly complex color combinations. The color search was also a completely separate interface from searching other product attributes and availability. It was neat, but it was not a great user experience.

It took some time to get back to the search by color functionality, but I've finally been able to get back to it and, with elasticsearch, significantly improve it.

Screenshot: colorized yarn

Color Quantification

One of the most difficult processes of supporting color searches is to figure out the colors in products. In our case, where we had thousands of items to "colorize", it would be easier to create an algorithm than have somebody manually pick out significant colors. When it comes to algorithms and research, the process is called color quantization. A lot of the inventory at the shop is yarn and, unfortunately, the tools I tried didn't do a good job at picking out the fiber colors (they would find significance in the numerous shadows or average colors).

Ultimately I ended up creating my own algorithm based on several strategies. In addition to finding the significant colors it also keeps track of their ratios making it easy to realize multi-color items vs items with accent colors. After batch processing inventory to bring colors up to date, I added hooks to ensure new images are processed for colors as they're uploaded.

Screenshot: colorized yarn Screenshot: colorized fabric

You can see it noticed the significant colors of the yarn and fabric above, along with their approximate ratios. With some types of items, it may be possible to infer additional meaning such as the "background color" of fabric.

Color Theory

When it comes to color, there are a few standard methods for measuring it. Probably the most familiar one from a web perspective is RGB. Unfortunately, RGB doesn't efficiently quantify the "color" or hue. For example, 244, 40, 5 and 244, 214, 214 are both obviously reddish, but the second color has high green and blue values yet the blue and green colors are not present.

A much better model for this is HSV (or HSL). The "color" (hue, H) cycles from 0 thru 360 where 0 and 360 are both red. The S for "saturation" ranges from 0 to 100 and describes how much "color" there is. Finally, the V for "value" (or B for "brightness") ranges from 0 to 100 and describes how bright or dark it is. Compare the following examples for a better idea:

  • 0, 70, 70
  • 0, 30, 70
  • 0, 70, 30
  • 0, 30, 30
  • 180, 70, 70
  • 180, 30, 70
  • 180, 70, 30
  • 180, 30, 30

Within elasticsearch we can easily map an object with the three color properties as integers:

{ "color" : {
    "properties" : {
      "h" : { "type" : "integer" },
      "s" : { "type" : "integer" },
      "v" : { "type" : "integer" } } } }


Elasticsearch will natively handle arrays of multiple colors, but color needs to become a nested mapping type in order to support realistic searches. For example, we could write a query looking for a dark blue, but unless it's a nested object the query could match items which have any sort of blue (color.h = 240) and any sort of dark (color.v < 50). To make color nested, we just have to add type = nested. Then we're able to write nested filters which will look like:

{ "nested" : {
    "path" : "color",
    "filter" : {
      "bool" : {
        "must" : [
          { "term" : { "color.h" : 240 } },
          { "range" : {
              "color.v" : { "lt" : 50 } } } ] } } } }

With the extra color proportion value mentioned earlier, we're also able to add a ratio range alongside h, s, and v. This will allow us to find items where blue is more of a dominant color (e.g. more than 80%). Another searchable fact which may be useful is color_count - then we would be able to find all solid-color products, or all dual-color products, or just any products with more than four significant colors.

While working on a frontend interface, I was having trouble faceting popular colors. A lot of dull colors were coming back. As a first step, I started using some terms aggregations with a value_script which created large buckets of colors from the h, s, and v tuple. That helped significantly, but then it seemed like there was a disproportionate number of very dark and very light colors. Instead of adding additional calculations to the aggregation during runtime, I decided to pre-compute the buckets that the colors should belong to. Now it's doing more advanced calculations and no runtime calculations. For example, all low-v colors will end up in a single bucket { h : 360 , s : 10 , v : 10 , ... }. Similar rules trim low-saturation colors and create the appropriate buckets for colors.


Given four key properties (hue, saturation, value, and color ratio), I needed a way to represent the searches from users. For searching individual colors, I settled on the following syntax:


This way, if a user is very specific about the dark blue they want, and they want at least 80% of the item to be blue, the color slug might look like: 80-100~190-100-50~10-5-5. Within the application, this gets translated into a filtered query. The filter part looks like:

{ "filter": {
    "and": [
      { "nested": {
          "path": "color",
          "filter": {
            "and": [
              { "range": {
                  "ratio": {
                    "gte": 80,
                    "lte": 100 } } },
              { "range": {
                  "h": {
                    "gte": 180,
                    "lte": 200 } } },
              { "range": {
                  "s": {
                    "gte": 95,
                    "lte": 100 } } },
              { "range": {
                  "v": {
                    "gte": 45,
                    "lte": 55 } } } ] } } } ] } }

The query part then becomes responsible for ranking using a basic calculation which roughly computes the distance between the requested color and the matched color. The function_score query currently looks like:

{ "function_score": {
    "boost_mode": "replace",
    "query": {
      "nested": {
        "path": "color",
        "query": {
          "function_score": {
            "score_mode": "multiply",
            "functions": [
              { "exp": {
                  "h": {
                    "origin": 190,
                    "offset": 2,
                    "scale": 4 } } },
              { "exp": {
                  "s": {
                    "origin": 100,
                    "offset": 4,
                    "scale": 8 } } },
              { "exp": {
                  "v": {
                    "origin": 50,
                    "offset": 4,
                    "scale": 8 } } },
              { "linear": {
                  "ratio": {
                    "origin": 100,
                    "offset": 5,
                    "scale": 10 } } } ] } },
        "score_mode": "sum" } },
    "functions": [
        { "script_score": { "script": "_score" } } ] } }

The _score can then be used in sorting to show the closest color matches first.

Screenshot: search screen shot

Of course, these color searches can be added alongside the other facet searches like product availability, attributes, and regular keyword searches.

User Interface

One of the more difficult tasks of the color search was to create a reasonable user interface to front the powerful capabilities. This initial version uses the same interface as a year ago, letting users pick from the available "color dots". Ultimately I hope to improve it with a more advanced, yet simple, Raphaël interface which would let them pick a specific color and say how picky they want to be. That goal requires a fair bit of time and learning though...


I'm excited to have the search by color functionality back. I'm even more excited about the possibilities of better, advanced user searches further down the road. After it gets used a bit more, I hope we can more prominently promote the color search functionality around the site. Elasticsearch has been an excellent tool for our product searching and it's exciting to continue expanding the role it takes in powering the website.