You can type nonsense words into this --- words for which Google will say no page in its entire corpus has a match --- and get color palettes back. What's it actually doing?
https://www.google.com/search?tbm=isch&q=sdlkfjsldkfjsldfkj
https://picular.co/sdlkfjsldkfjsldfkj
If you put it in quotes, though, Google gives no results and Picular crashes (500 internal server error from an XHR):
https://www.google.com/search?tbm=isch&q="sdlkfjsldkfjsldfkj...
My guess is you're still doing a Google Image search, but when it returns no results, you return something random and cache this query for consistency. Is that about right?.
I would not really call that machine learning, since it's basically an image search engine with a color filter.
I wonder if it's just automatically accepting the spelling suggestion Google provides for searches?
Perhaps custom search displays differing results?
The fact that it works for non existing search results shows that it's ML
If you try linking to https://picular.co/four+letter+words
...some of those four letter words won't return results, even if they do exist. Probably because they (futurememories.se) don't want to associate their product with lewd profanity, if/when people start sharing random links on social media.