TrustExecutor I'm pretty sure there is no color, nor ID, in the overlay popup while active on the phone. In the fullscreen incoming call I have noticed that the ID (SIM name) is displayed in the same color as the background which is barely visible.
Edit: It could be white text over the fullscreen background color, probably since I'm on Dark theme.
However it works now I don't find it clear enough for me to feel comfortable enough whether I'm answering a work or a private call. Thinking further a lot of people are color blind. Maybe an optional setting as Accessibility could be a compromize, +making it visual in the overlay pop while active on the phone.
It is a security matter being confident which channel you are responding on (and how you respond).