A buttercup is a "solid" object and thus radiates a thermal blackbody spectrum. However, a flower isn't hot enough to radiate significantly in the visible part of the
spectrum (it does radiate in the infrared, however). Like most normal objects, it is seen visibly by reflection. If illuminated by white light (e.g., sunlight), the visible
spectrum of a buttercup will be as shown above. The blue portion of the illuminating light is absorbed by the flower so that only the green and red parts are reflected. As green and
red combine to make "yellow", the buttercup appears yellow visually.
Spectrum of Meadow Buttercup