I wouldn't imagine that color change would require a primer. The gray under the taupe might muddy the final appearance a bit. Sorry to throw a monkey wrench in your certainty. If you already have the gray, and some time, in an out of the way area, do a small square of primer and then do a larger sqaure of finish to see how the final color looks over the yellow and gray. Keep us posted.
I now understood the importance of the paint stores color charts, on the back side, there's a chart which tells the optimum color results using the designated Color-Prime and also LRV, light reflective value.
My entire life I've just slapped a coat of paint on the wall. But until now, I've never painted with strong colors.
Also, I'm really, really happy that my paint store is having a sale this weekend and I'll buy all the paint I need and just take it back to be shaken as I use it.