What makes a tulip come up one color (even if mixed), but it doesn't change even if the soil is acid or not?

Most flowers won't change color due to the pH of the soil (the amount of acidity or alkalinity) although hydrangea, morning glory, and petunia will to some extent. The article I sourced below gives a basic overview of the work being done to produce a blue rose (call me when they have that; I want five).

To do so, they have to alter the pH within the rose petal cells themselves to produce a blue color. They do that by tinkering with the rose's genes to alter the amount of acids and bases picked up and exuded from the roots and retained in the plant. Most plants hold their pH within fairly tight boundaries regardless of soil pH and the color of the flower doesn't change.

Interestingly, pure white hydrangea can't be easily color-changed and usually only manage a very gentle pink or purple as the flowers age. Only pink and blue hydrangea can be turned to the other color, or to a gentle ivory white at a neutral pH.

While not specific to tulips, the same information does apply as they also use flavonoids to color their petals. Tulips are currently available in a lot of stunning colors except a true blue. About the best you can do there is a lilac or purple. Most of us who love tulips (I have about 1,500) would also love a blue tulip. So far, no.

Tulips wont change color for anything but viruses. Viruses can change the colors over time. Mostly aphid borne viruses. The color is what it is, genetically speaking. The PH level doesn't affect the color at all, only the health. Hope this helps.

Not all flowers are affected by acid. It also may have something to do w/ the fact that tulips grow from bulbs and hydrangeas grow from seeds.

I think it is impossible to mix purple with out blue or red. I would suggest using different shades of blue and red. Or add complementary colors like yellow for a dulling effect. Use more red than blue, more blue than red. Add diffenet colors if you want. Add white for a lilac color. And avoid black at all costs. (EDIT Black makes your color seem like a black hole, use yellow for dulling, or a dark blue or dark green) And it would help to know what kind of paints. Acrylic, oil, watercolor...?