Is there a computer program that tells you how to mix colors in proportion such as two parts of white plus 1 part of black plus 1 part of blue and show the resulting color?

There are so many Excellent programs out there. If you type "mixing colors" in your search box, you can pick and choose the one that is right for you. I have previewed a couple of them for you and provided the links, to get you started.
See the Related Links for "how to mix colors properly" to the bottom for the answer. See the Related Links for "color mixing" to the bottom for the answer.