How to color merge commit when use pretty format?

I have already the pretty format log oneline like this

g help ll
'll' is aliased to 'log --oneline --first-parent --pretty='%C(yellow)%h %C(cyan)%ai %Cblue%an%C(auto)%d %Creset%s''

I want to print more pretty to color the merge commit msg specially. How can I get to it ?

Thanks.