If you use the normal Colemak layout then  you can use Alt Gr with n, c, r, t, and d to get: ñ, ç, `, ´, ¨
I haven't taken a look at the spanish Colemak versions so I can't say anything about them, but if you search for them in the forum you'll find some information.

Edit: I forgot to mention about the spanish keyboard, the only differences seem to be that the key that has <> in the spanish (to the left of z) now will have -_ and the key that has the ç  now will have \| No problem there, really. A bigger problem is that the english and US layouts have the +, -,  =, &,... on different positions to the Spain layouts keyboards and that can take some practice to get used to.