I was just working on this exact issue last night, was going to post in our Initial Contrast Experiments, but let me help you with this question 1st. It's a good question.
Previously, I was previewing Contrast paints at the store, I had an "oops" moment where I I didn't distribute it evenly enough on a Primaris pauldron. Notice that splotchy dark bit halfway to the purity seal:
I was going to finish the model at home with normal paints anyway, that was always my plan, mainly to paint areas I left blank, like the chest aquila. Initially I wasn't going to paint over any Contrast areas, because a main reason to use Contrast was to speedpaint to get it battle-ready...but that one shoulderpad really bugged me.
So I went ahead and tried to not only directly paint over that shoulderpad, but to fix it so that it's hard to tell where the Contrast paint ended and normal paint started:
If I pointed to you the exact spot where I applied normal paint over Contrast...like I just did right now, yeah, I think you can notice there's a difference. The Contrast paint was Militarum Green. The normal paint was Vallejo's Russian Uniform Green. I absolutely thinned it a bit and carefully applied it, that took a minute or 2 (which is actually a long time considering I had painted this model up under 5 minutes with Contrast).
That was the best "proof in the pudding" moment, where I had to match Contrast with normal paints because I had an accident. It wasn't intentional, I wasn't out to test a theory out, but it provided actual evidence that Contrast paints and normal paints worked together fine. I seriously just eyeballed it, turned out okay.
And it wasn't the only place where I painted over Contrast. That bolt rifle was painted with Contrast Black Templar Black. I then drybrushed over it with normal paint to bring out the details, perfectly normal.