Glandular trichomes (plant hairs) produce special liquid chemicals that are secreted or vaporized from their tips. Most chemicals made within trichomes are terpenoids, which are not only odoriferous or fragrant, but also may be colored, e.g., red, orange, or brown. Hence, presence of glandular trichomes in some cases modifies leaf color.
In general, it can be assumed that the function of the glandular trichome has to do with plant defense, because the chemicals by and large are produced to discourage herbivores from eating the leaf.
When leaves are strongly pigmented with vacuolar pigments, trichomes may also produce these compounds and thereby influence leaf color.