All right, I suppose many of you will come in expecting questions about curves' vertexes, and how to texture them, or something. Well, actually, I want to know how to change the color of the curves on the 2d grid. They turn red, like any normal brush when I select them. But, whenever I deselect them, they turn black, against the BLACK and GREEN background. So sometimes, its hard to detect them. Could somebody point me in the right direction please?

Have you set the curve color to something different than the default? As far as I remember, only func_groups were disturbingly black in the black and green background (that I also use, btw), but of course it might be that I don't just recall it anymore. Maybe also the minor grids, because I seem to remember messing around with those colours.

Yeah, some colouring you can change in Radiant menus, the others in sp_entities.def and mp_entities.def that you can find in your base/script folder. I suggest making the changes in both of them (and make back-ups before screwing them up). Do the changes when Radiant is not running. They should take effect when you start it after that.

Yeah you can change colors if you want to. I think it's under preferences.

To be exact colours can be changed in the Misc \ Colors menu in Radiant but there's no separate option for patches.

Does this happen with a newly created patches or only after you capped or thickened them? In the former cases the reason is that the result of those function is a func_group by default which are - as Lassev said - always black. This can be changed only in the SP and MP entities.def files as - again - Lassev said.