I've been toying with ideas on how to put emblems on my armor when it gets to that point. One method I'm thinking of would be a sort of stencil method. For the sake of example I'll use the emblem I've chosen in H3, ODST, and Reach: the howling wolf (white wolf, black background) with the octogon border (blue).
Step 1:Find a high enough resolution image to be able to print out a template scaled to the size you want the emblem to be on the armor.
Step 2: There are three colors, so we'd print out three copies of the image (if they'll all fit on one sheet of paper, all the better). Even though this might work if printed on cardstock I'd suggest just printing it out on regular paper and then gluing it to the unprinted side of a thin cardboard box (for example, a ceral box, or the 12-24 pack soda boxes).
Step 3: Carefully cut out the "negative space" in each template with an xacto or razor blade, being very careful not to knick the edges too much. What I mean by negative space is the area that will be filled with paint. For the example I'm using, one template would be cut out completely to give us the octogon shape for the blue layer. On the second template, cut out the circle and wolf, leaving the octogon in place so you have a circle for the black layer. And on the third template, just cut out the wolf so you have a shape for the white layer.
Step 4: You'll want to have a reference point on the armor to mark the center of the emblem to help with lining up the subsequent layers. Tape down the first template where you want the emblem to be, making sure that it is pressed as tightly as possible against the armor to keep the edges as smooth and crisp as possible. Be sure to mask the surrounding area as well to prevent overspray. For this example, you would then lay down a light coat of blue paint. You'll want to use very light coats and let them dry before adding another here to keep it from running or dripping, which would lead to a sloppy paint job. This will take patience, slowly adding one coat at a time until you have the desired coverage.
Step 5: Once the first layer of color is completely dry, carefully remove the first stencil and line up the second. You're going to repeat the same process of taping it down, masking the surrounding area, and slowly building up the next coat of paint. You'll want to check and check again to make sure you have the template lined up and centered, unless you want to run the risk of having to either start over, or be left with a lop-sided graphic.
Step 6: Repeat the process again with the third and final layer. The wolf head emblem I'm using for the example has an eye spot, which would mean either having to make a fourth template with just the eye cut out, or to save time put a small piece of masking tape on the armor, line up the template, trace where it meets the tape for reference, and then draw in the eye. Using the xacto/razor blade, very carefully (and lightly) trace around the outline of the eye. You don't want to press too hard your you'll cut into the paint you just put down. Once you've seperated the eye, carefully remove the rest of the tape, leaving just the eye spot behind, then put the template in place. Once you're done painting the final layer and it has dried, slowly peel off the eye tape.
I realize I sometimes have a roundabout way of talking, so if it would help to have a visual tutorial just let me know and I'll whip something up.
How well this method works depends on the complexity of the symbol you want to put on the helmet. If there are a lot of intricate shapes, details, or multiple colors, a better method might be to print it out as a decal if you have access to a printer capable of doing so. If there's a Staples or other copy/print center nearby, you might be able to get them to do it for you. If you're really lucky you might have a custom print shop in the area where you could get a more weather-worthy decal printed up so you don't have to worry about temperatures or moisture causing it to peel off. Although, if you put the decal on before you do the clear coat, that might seal it in enough to keep it from peeling.