The secret to mixing cream and white is to have a little bit of the the cream tone in the white. So if you look at the light end of a paint deck and choose the top color as your white and the next one down the page as your cream, you can rest assured that they'll live happily ever after. Of course, it's not always as easy as that, but that's the essence of it.

As for your cream and grey bedspread clashing with your white sheets, I suspect the grey in your sheets is a very warm grey, not a cool bluish grey. And the white in the sheets is probably not a warm white, but a very cool white. So the warm grey and the cool white are not good friends and will not be happy together because of one being cool and the other being warm. Clear as mud?? Well, it's great that you noticed the problem and asked the question. Curiosity will get you far in your design studies.
Carolyn Albert-Kincl, ASID