how can you paint a good green lantern symbol?

whats a decal?

You can use a decal if you don't want to paint it.

Take a piece of masking tape and draw the symbol on it, then stick it on the figure where you want the symbol. Cut out the white parts of the symbol with an exacto knife, and put a coat of green paint that matches the paint already on the figure over the area you just cut out and on the masking tape. That creates a seal so the white paint won't bleed through. Lastly paint the white parts and remove the tape.