dust DC 540 over it first before yellow

 First, welcome aboard Ben, I see this is your first post.

 Second, keep in mind that any yellow might just be about the worst color for covering anything. Even over white it will usually take multiple coats to get uniform coverage. In short, painting yellow can suck.

 What you want to do is possible but, like Dave says, you'll need a basecoat of a lighter color for the yellow. Brodak colors have very good pigmentation which helps a lot. Using Brodak dope I've definitely pulled off some "tricks" that some would say can't be done. I once forgot to add a graphic stripe on the lower rear fuselage of a Super Combat Streak model. I had done that particular model with an overall white base as a start, adding the orange and black over the white. When I painted the orange I forgot to mask for the lower rear fuselage "strake" stripe. With nothing to lose I tried spraying that stripe using Insignia White directly over the Cruiser Orange. Using an airbrush I got it to cover just fine. I will attach a photo of the 'Streak in this reply.
 
 For this "trick" I use a cheap $40 Badger airbrush designed for small plastic models. I run mine at 30psi. I've found that the nozzle opening size will largely dictate your usable dope/thinner ratio. I use a medium nozzle and start with at least a 50/50 mix but many times end up adding even more thinner to get things flowing. By using the small airbrush it allows you more control and you can spray the initial passes "dry" as you gradually build up the coverage and color. The airbrush also puts on less paint, helping you to save weight. Just tonight I just finished a paint scheme using Brodak Stinson Maroon as the base/primary color for the model, and then Diana Cream for all of the other trim work. Once the maroon base color was on the model all other trim work was done using only the small airbrush. I will be posting photos of that model here soon.

 In your case this process will work, but you will still definitely need a white or light grey base for your yellow. Keep in mind that the yellow will be a slightly darker or muted shade if you use anything other than white for a base.

 Hope this helps!

I too use an air brush for trim and graphic work.   Have a Harbor freight gun for doing the base coats and sometimes final clear.  The Streak looks great.   

It's more work but you can back mask the trim areas and then not have to spray over the red.  It may save a bit of weight also.

James

DC-540 is a light grey and covers easy. Sands easy and is light. Best stuff I have ever used. I buy it 6 cans at a time. I'm still waiting for it to be discontinued as is anything that works.

Both red & yellow are transparent.

You need to paint WHITE first, then red or yellow.  This is the the fine print of Rndolph color dope products.  I always comply with this and it works fine.