Would bishops be stronger or weaker on the same coloured squares? Assuming one or both players makes a mistake and transposes a bishop and the adjacent knight when setting up the board, would this help or hinder them?
The answers to this similar question suggest it's a disadvantage. However using the lichess board editor for different variations of this (one vs both sides with both on light squared or dark squares, etc) generally shows a larger starting advantage for white for all variations in the stockfish evaluation (up to +0.8 for white having 2 dark-squared and black 2 light-squared, and +0.2 for black being normal but white having 2 dark-squared compared to +0.1 for the standard start), but I'm not sure how valid this analysis is for an alternate starting position.
And the correct answer, as always, is: &quot;it depends on the position&quot; :-)
OK, assume that you start with Bb1/Nc1. Lichess says +0.1. But this is the sum of...

the N generally standing a bit ugly on c1/f1, from my positional feeling
the B doing some concrete attacking (Stockfish unsubtly starts with c3)
other random concrete aspects
the question you asked: What is the isolated effect?

The isolated effect, of course, is that you are a whole piece up on all white squares, but down on black squares. GM Bangiev would rejoice: Adapt your strategy to be playing your dominance on white. (If that is possible.) For example, Bc4+Ng5 against short castling is completely useless. Fire it up with an additional Bb3 and Black already has a big problem.
Remember that different-colored bishops tend to be an advantage for the attacker, who exactly enacts the strategy I just described. (As the dude with the pair I of course would try the exactly opposite strategy - trying to get a position where both of your bishop just stare at pawns posteriors.)