Exterior balconies have a fiberglass deck - 1 solid piece, as well as the stair treads
They were original blue grey colour, but I want to change to cream color, so need to know what paint adheres well to fibreglass for high traffic area

I assume you mean a composite deck. Different manufactures different composite materials to make their decking, but none are fiberglass. As far as I know none are praintable. There are no pours in the material and therefore the paint will not adhere very well. If it sticks at all, it will wear off very quickly under foot traffic.

With that said, Krylon makes a spray can that claims to adhere to plastic. I seriously doubt that it would stand up to foot traffic. I would certainly do a test area before using it on an entire deck.