Tess - it’s difficult to give advise without seeing the sort of
piece you have in mind, but here goes…
I can’t think of any reason why those colour wouldn’t work on
18k. The only word of caution would be regarding using castings.
Imperfections in the casting can cause enamel to “blow” leaving
pin-pricks in the enamel.
Correct. You can make colours darker by carving more deeply but
have to watch that the depth difference isn’t too dramatic since
this can cause cracking (particularly with fine silver).
Alternatively, you can use flux (transparent enamel) on all or parts
of the piece either above or below your chosen colour.
Enamel will bond to a perfectly flat surface as long as the metal
is completely clean.
I prefer to have all findings soldered before enamelling (using
hard solder). Some use easy solder after enamelling but I think the
"disaster risk" is higher.
As enamel is only glass the surface will simply crack if you try
to carve it. After the final firing the surface should be perfectly
glossly. Any tiny imperfections can be buffed out on a wheel using
pumice powder & lots of water. If you prefer a matt surface, the
piece can be dunked in matting salts for a couple of minutes.
Hope this helps, Deborah