Yes, you can paint exposed b-vent to match the color of the siding or roof shingles. Follow these simple guidelines:
Scuff and clean the surface to be painted with steel wool. Clean dust, oils and fingerprints off the surface using a household degreaser cleaner.
Locate a reputable paint manufacturer/supplier (Rustoleum, Sherwin Williams) to review their recommendations for painting galvanized steel that could get to possibly 250 degrees F at the extreme.