Not completely wasted. I do a lot of work with Decals on 1/600th aircraft which are a similar (in decal sizes) to what you are trying to achieve.
If you scroll to the bottom of the link below you will see how I used clear decal paper over a bright colour to tidy up the edgeshttp://raidscalegaming.pbworks.com/w/page/30016942/Arab%20Israeli%20Aircraft
For your purposes for Yellow over Red:
paint a white area on the model.
Make up your decal on clear paper using yellow and red BUT put a border around the decal in the base colour of your model (remember to keep the border quite thick)
Trim your decal and place over the white area. The White will allow the Yellow/White letters to show through with the correct luminosity.
Paint the base coat of your vehicle back up to the edge of the decal border covering over the white
Not a straightforward as using white decal paper but will let you use what you have already