I think it’s based on what you want. For my wedding the mother’s will wear whatever color they want and what they feel best in. I don’t want them matching the bridesmaids though.

As Miss Manners would say, this is a wedding, not a costume party -- so everyone including your mother and MIL should wear whatever they think is appropriate per the attire you put on the invitation. If you want a costume party then you need to be abundantly clear in the invitations and with your wedding party that they are to wear certain colors and shades of those colors.

My wedding colors were like a wine color with hints of silver, pink and green. The bridesmaids wore the wine color and my mother wore silver. My mother in law wore Navy blue. I just let them wear what they wanted to wear. I did, however say no ivory, white or colors in that family.

When I planned my first wedding, our colors were royal blue and light grey. I told my mother she could wear whatever variation of blue or grey she wanted, as long as the dress didn't closely resemble the bridesmaids. We told the same thing to my ex's mother. My next wedding we'll basically say the same thing. We want our parents to match somewhat, but the rest of the family/guests can wear whatever.

Let her wear what every she wants.

I know my aunt wore the same color as us bridesmaids for my cousins wedding. I personally am planning tye dye so I prefer my mom to wear purple and my dad to wear blue but if they aren’t comfortable like that then they can wear whatever they want lol