Color APIs
The color.hpp header provides two classes for performing color management, interpolation, and conversion - espp::Rgb and espp::Hsv which can be converted between each other. The RGB color space provides support for additive blending (which includes averaging, as opposed to light-model-based mixing) and is therefore suited for producing gradients.
Please see Computer Graphics and Geometric Modeling: Implementation and Algorithms, specifically section 8.6 for more information.