Yup, red is the worst for getting good coverage. I recommend a grey undercoat, unless you want some white on the final finish, then I'd use white as the undercoat. It'll be tough to brush out evenly, because not matter how long you let dope dry, subsequent coats dissolve the coats underneath. So light spray coats look the best.

Brushing can be done decently, though. Here's what I recommend:

Sand the balsa thoroughly with a sanding block and progress to the finest grit you got ... hopefully 300 or so. Hopefully you can sand off most of the red you've already put on.

Brush on a coat of grey, do the whole plane (right over the red you've already done).

Sand again. Sand off most of the grey, so the color is only left in the low spots and in the grain of the wood. Always use a sanding block!

Brush on another coat of grey. Hopefully this coat will look even and pretty smooth when dry. Don't try to fill up all the grain in the wood ... it would take lots of dope and make the plane heavy. But you may want to sand again and paint grey again ... that would be three coats of grey, but most sanded off between coats.

When the grey is pretty smooth and even, brush on a nice thick coat of red. Use a wide brush, full of paint, and just lay it down. Try to avoid going back and brushing it out too much. Keep the brush wet with paint. That should minimize the bleed through of the grey and give a nice even color. Oh yeah, and as Jim said, make sure you mix the colors thoroughly. Red really separates out.

Good luck,

Phil