In the curves function, select red. Drag the curve up a bit in the middle. Then select green, drag the curve down a bit in the middle. Then select blue, and pull the far right point all the way down.
NB this is only one way of doing it. You will no doubt be bombarded with >5 alternative methods, and at least 3 people telling you to do exactly what I've just told you. You may also have started a lengthy discussion on whether it is appropriate to colour it at all, or if we should render it in H-alpha red.
A good alternative is 'Dave's Solar System Recorder'. I don't use it, but it will take multiple exposures, and do the colouring on the fly for you.
Hope it helps!
Andrew.
