Is it bad form for her to wear cream to the wedding given that she is going to be wearing much the same colour as me. My friends are divided on this issue some are scandlised others couldn't give a monkey's..
I wouldn't have thought so, but just my opinion. If no one could wear white but the bride then mens shirts would go out the window (if you see what i mean). Now if she wears a veil......
You can assess your present - and indeed future - relationship with your partner's mum quite easily - can you have a chat about colours of outfits over a coffee? If so, you can agree a solution quite easily in advance of the day. If, as is often the case, you're just about on speaking terms, you can wear what you like without risk of offending her - she'll be offended what ever you do, and you probably won't care!
Some people have a colour code at weddings now, so if it's not too late, tell everyone that you want them to wear colours that go with your colour scheme.
if it offends you, and its too late or she won't change her mind - a wee glass of red wine at the start of dinner should see it right and subtly change her colour scheme! :]