It's normally best to paint lightest colours first. So, paint the underside and, when dry, mask this where needed and paint the Dark Earth.
Rather than using templates, or masking, you may want to try what I normally do. That is, once the Dark Earth has fully dried, mark the camouflage pattern in pencil, and include the letters 'DG' here and there inside the areas to be painted Dark Green - that way, you'll know which areas are which!
Then either brush paint or airbrush the green, starting by following the pencil lines, and then filling-in the main areas. Even with a 'only relatively' fine line airbrush, if close in, at low pressure (about 15 psi), this is not as hard to do, free hand, as you'd imagine, and much easier and less hassle than the b@ll ache of complex masking, or trying to keep templates in place on a relatively small surface area. As a hard edge demarcation is required in this instance, any slight feathering or over-spray which might occur can be tidied-up with a paint brush.
If you feel you'd rather mask the entire underside, to prevent over-spray, then just mask the required edges, filling the rest of the area with cling film, tissue or kitchen foil, held in place bt the edges of the main masking tape - saves time, and saves on relatively expensive tape!