[clbecker]

That looks pretty cool. As far as scaling issues went, the bulk of them were addressed just by using Solr on pretty beefy hardware (for our full library of 30+ million images, we're running on Dell r510s w/ 24 cores and 128GB Ram). Of course, depending on your hardware limitations there's compromises you can make to increase speed / reduce memory usage - i.e. the fewer fields you need to index and sort on the better & the lower the cardinality of each field, the better too. Also, since our input only consisted of one slider, we were able to run all the distance calculations beforehand, and just store a score that represented the image's distance from the given slider position - something like that might also work for you, since you're using a limited palette of input colors too.