If you put gyproc joint filler over the screw-heads,and then put a tape down each joint in the plasterboard, again using the joint filler,this will give you a decent enough job to paint if you are pushed for time.Normally,the tapes on the joints and the nailheads,once filled, would be finished off with joint finish and then sanded down.To be honest,and I say this to people on jobs,there is more hassle with taping, than plastering.With plastering, it's all over in one go,with no sanding down.(or there shouldn't be).It might take a little longer to dry out that's all.
Again,referring back to your query on colours of sand.There are different colours and it probably depends on the area where it is quarried.But I think you are talking about colour that can be added to mortar, to give it a specific colour.You can buy different colour powder dyes from builders merchants.Regarding your last point,I don't know whether yellow sand gives mortar a more grey colour,maybe.Perhaps someone else will know.
Roughcaster.