How can I cover up my pimples without using makeup?

Visine Eye Drops. Visine -- "it takes the red out" of more than just eyes, by constricting dilated blood vessels that cause redness. Use a q-tip to apply a few drops to your pimple to temporarily decrease redness. Aside from traditional "makeup" concealers, there are "tinted" acne spot treatments & green-tinted products (often used for rosacea) for hiding redness. Be sure to use "non-comedogenic" products.
Tinted sunscreen . Might be worth trying a tinted sunscreen or tinted moisturizer or check with a medical aesthetician in your area.