Fix #132

[SarahOuologuem]

Hi! I've encountered an issue when running mu.pl.scatter (#132) , as sc.pl.scatter() fails when passing the color as a list, as e.g. described in scverse/scanpy#1986 To avoid this issue, my suggested changes only take a string as input for the color instead of a string or a list.

[bio-la]

Hi @gtca, can you help with this issue? thanks!