Recently, I discovered a problem with the color picker I was using for the gradient slider I wrote. Apparently, it does not cover 100% of the color spectrum. So I started googling and found some algorithms on Wikipedia on calculating color gradients. So after a couple hours it was perfect, at least in terms of having all the colors.
The downside is that the slider is now much wider, so it requires more room in a processing sketch. But I can say that the color gains outweigh the width concerns.
Here’s a picture of the same sketch as before with the new slider added: