Lozenge

What color should I airbrush on first?

I would airbrush the upper surfaces the same olive color as the forward panels and cowling. You might want to airbrush the under surfaces a lighter color, perhaps a CDL type color or even light blue depending on the predominant color of your lozenge, but I'm not sure it really makes that much difference. If you're using the Eduard lozenge, it's fairly opaque so just about any medium hue would do. If you're going to mask over the lozenge decals, be sure you give them a good coat of Future, other wise they'll peel right off when you remove the mask. Also use a low adhesion mask.

One other point, be sure to seal the lozenge and rib tapes good before applying the crosses. On mine, I didn't do that, and the rib tapes "bled" through the white trim on the crosses. I think the decal solvent on the crosses actually melted the rib tapes a little. For a little more accuracy, I would recommend using Aeromaster blue rib tapes instead of the purple ones supplied by Eduard. The pink kit tapes are a little dark, but close enough.

Excerpt from the WWI Modeling List Archives from a post by Otis Goodin.

Have people been successful masking over lozenge to spray?

Do your painting first. If you have to mask, use wet newspaper. It will stick without being sticky.

Excerpt from the WWI Modeling List Archives from a post by Tom Cleaver.