Our home is 30 years old and the paint on the walls are semi gloss. Can I use fairly new ceiling paint as a primer. Walls are blue and white and I want to paint them green

A primer paint is a fairly low quality, cheaper paint used on previously unpainted surfaces. Some might argue that it has some qualities of better adhesion. Its primary purpose is a cheaper paint that is used to seal the surface and reduce the amount of paint necessary for the final coat.

If the surface has been previously painted you don't need a primer coat.

You could use the ceiling paint but you will probably still have to apply two coats of the green to get good coverage.

I agree with Harold unless u are trying to cover water stains or a similar blemish. Then u would need a stain blocker/primer.

Chuck