For pastels, I always use a colour a little bit lighter than the one used to paint the kit. You can use Tamiya pastels, or MiG pigments, or pastels bought from art shops (kind of chalks that you have to reduce in powder).
If the problem is due to air bubble under the decal, you can try this: to make little holes with a needle in the bubble, then to use MicroSol. Not 100% guaranteed, but gives good results many times...