Short post today, but have you ever had the right decal in the wrong color? For example, the Blood Angel's symbol in white for your Death Company Dreadnought, but all the SM decals have either yellow or black? Fret not, because the solution is simple:
Get some thin plasticard, apply the decal, use the decal as a guide and cut it out.
And there you have it, a quick stencil, now you can have the same shape as the decal in any color you wish. Obviously doesn't work right away for multi colored decals (you'd need to make a separate one for each), and you'd need masks for those that have uncoloured centers, like the eyes of a skull, but this is good enough for most jobs. And the obligatory picture: