I don't think just adding a couple more coats of candy over the graphics will get you what you want. If you are looking for a 'darker' shade, then just make the base darker.
Spray the silver, mask off your graphics, slightly darken the silver, then spray the candy over the complete panel.
THEN do the marble over the graphics...of course' you'll need to remask the graphics.
OR, you can spray the silver, mask off the graphics, do the darker silver color. Do the marble over the graphics, then remove masking, then spray the complete panel in candy red. The only downfall of this is the marble will be under the candy. Not a bad thing though.
I've done quite a few jobs where silver is the base, then I do black 'marble' effect over the silver. Then I candy over that. I have a step by step in the How-to section, but you need to be a subscriber.
If you did it this way, you should add an outline around the graphics.
Here is a pic where the base is the marble effect.
