I expect that there would be a mixture of colors in the tomatoes produced by the seed. Then one would save the seed from the desired color, which would also the following year produced various colors. Then one would save only the color desired, etc.
There are mathematical formulas for predicting results, if you know the genetics.i.e.: 1/2 one color, 1/4 another color, 1/4 yet another color.

A very good book about this in simple language is: "Growing Your Own Garden Varieties" by Carol Deppe I think that's the right Title and Author.

Anyway, I'm sure you can find the book at Amazon.com or maybe try "Bookfinders". It's a very good book for "us beginners"

Best wishes to you for Happy Holidays,
Junkmanme

Also, you should note that tomatoes generally fertilize themselves before the blossoms open, so the cross may be minimal at best.

Again...Happy Holidays!
Junkmanme

If the red parent is a stable inbred or a hybrid that doesn't have have a mix for colors in it's parentage, the first generation of seedlings will be red. Red is dominant over yellow in tomatoes.

If you allow the seedlings from your cross to pollinate themselves, their seedlings will be the mix of colors that that Junkmanme gave you, but in this case maybe 3/4 red and 1/4 yellow if the red totally dominates the yellow.

Junkmanme was right about the tomatoes fertizing themselves as well. You'll need to remove the anthers before the flowers open.