Cool idea. The implementation could be improved - you can only play it once a day, and it's simply too easy.

Even in hard mode, without seeing the reference colour, you can find any number with five guesses (and the 5th guess is only necessary if there's an F). Pictures related:

I wish you could have unlimited guesses, and only had to get kinda close, but also only had the color to go by. Maybe “how close can you get in 5 guesses”, rather than binary searching for characters. Also, this reminds me of sha256le.