Pollinators (animals that feed themselves by visiting nectar-producing flowers) would rather not compete over the flowers they visit. So, during the hundreds if not thousands of years that they've been visiting plants in search of nectar, they and the flowers have worked out a sort of deal: Whereas some flowers are specially suited to butterflies, others are specially suited to hummingbirds (others to moths, others to very small flying insects, and some even to bats).
So why would a hummingbird flower be red? Because insects don't see red very well.
Because insects don't see red well, they're more likely to skip red flowers, leaving more nectar in the red flowers for the hummingbirds to find. In contrast, hummingbirds do see red well. So a red flower is a sign that says to a hummingbird, "Not many insects have been here. More food for YOU!"
Color isn't the only way hummingbird plants discourage insect visitors in order to attract more hummers. Many hummingbird plants have flowers that are long and tube-shaped and that dangle downward. A hovering hummingbird, with its long tongue, can get nectar out of one of these flowers. Many insects cannot.