I want to ask my brother in law's girlfriend to not wear black to our wedding.
What is it about black that you don't like?
It relates to death and funerals.
You bil's gf wearing black to your wedding is not the end of the world. You telling her what to wear, however, is not correct. Let her wear what she wants and deal with your own pickiness instead.
I'll think about it.
I'm glad you will consider her decision. It is your wedding after all and you get to make the decisions, but leniency goes a long way. Perhaps you could come to a compromise and ask her if there are any other colors she would like to wear.