I've sanded it all down and filled

If I use a basic base coat white then the colour I want will it be okay and not show?

I want to paint it grey light.

I suggest you treat the prepared surface with a solution of 5 parts water to 1 part PVA. Allow to dry completely, then test coat a small area first. If the result is positive complete all the walls. If you are using a good quality emulsion there should not be a need for a base coat.

Good luck

I've sanded it all down and filled

If I use a basic base coat white then the colour I want will it be okay and not show?

I want to paint it grey light.