Just found a little gem on the GrapheneOS forums...

Do you disklike the pink screen when receiving a call? You can change the color...

Settings > Network & internet > SIMs > your carrier > pencil at the top right

#GrapheneOS

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Mine was cyan for SIM1 and use to be blue for SIM2 but changed to cyan as well so I didn't know at first look from which SIM I got a call.

Now I make one pink and one orange, much better!

Thanks for the tips.