Is there any reason to look at the target instead of just going for binary search? The least significant digit is kinda meaningless to guess without fixing the more significant one.
For instance, my initial guess was off by (+1,-1,-1,+3,-1-2) and my first impulse was to look at the target and see that I had too much red and not enough green.
*standard HN disclaimer
More directly: it's actually pretty fun to guess these straight up. disclaimer: I have a strong bias towards having fun with color stuff - c.f. https://material.io/blog/science-of-color-design.
RGB is neigh-meaningless, true. Here, I get to exercise my intuition I should balance between red and blue based on warmth via hue of the color. Then, jack up G to match the lower of the two if its a light color / has high tone.
Noughts and crosses? It's non-trivial if you're 8. No fancy algorithms needed. The other examples you mentioned can't be solved by an 8 year old in their heads.
> do they have a Stockfish?
No, which is why chess is still fun. You don't need a computer or even a pen and paper to solve noughts and crosses, or this.