I always disable it. Having everything using the same exact colour palette is boring/homogenized. The themed icons are even worse, simultaneously killing iconography art and making it much more difficult to differentiate between apps in the app drawer quickly.
I don’t think they are different? Here’s what the phone, messaging, and contacts app icons look like on my pixel 8.
Edit: I’m a dummy. I just realized you meant the contacts’ icons in each of those apps. I just confirmed those colors can vary between each app. I agree, that does seem like an oversight.