Deano,
Once the yellowed decals are on the model, I'm not sure that problem can be overcome.
Anyone have a fix for that?
Use the tip of placening the decals for a week or so taped to a window facing the sun, or if you have a steady hand, use a new no.11 blade and trim away the clear carrier from the edges of each decal - but it's hard to do for things like stencilling.
If it's yellow on the model, you might have to live with it..............
Cheers, Tree