There are two types of gelcoat that you can buy.
Oxidation causes the surface of old gelcoat to become white and powdery looking. This can be removed with a polishing compound.
Gelcoat can be coloured to achieve almost any colour. The tints come in little tubes and one drop goes a long way. Like paint, gelcoat comes in both neutral base and white base formulations. The final colour is dependent on which base is used. Consult the tint's colour chart to determine which base to use to achieve the colour you want.
Why use gelcoat?
That's a good question. Composite boats have been made with gelcoat for decades. While conventional, it is not necessarily the best solution. Gel coat is cheap, but it is not particularly strong. It contributes little to the kayak other than to protect the underlying resins from UV radiation. For the weight, replacing the gelcoat with a thin layer of fiberglass and tough paint would provide more strength and durability than gelcoat. The extra fiberglass would be quite abrasion resistant and the paint would be very easy to fix when scratched.
Some racing boats do not use gelcoat. Racing sculls can have a carbon fiber hull painted with an epoxy paint.
How do you fix gelcoat?
See Gelcoat Repair