Flowers are the means by which plants attract pollinators. Pollinations leads to fertilisation and fertilisation leads on to seed formation and the propagation of the species. For plants, like sunflowers, the pollinators are insects - so the plant displays something bold and eye catching for them. However, the brilliant yellow and orange colours that we see are not what an insect sees or is attracted by. Insect eyes (compound eyes) see the world very differently - one key difference is that unlike us, insects can see ultra-violet light. Sunflowers (and many other plants) take advantage of this fact by incorporating UV absorbing pigments in their structure; so what we see as a ring of colour with a darker centre is for insects a more complex set of of rings.
The flower of a sunflower is, in fact, made up of hundreds of tiny flowers - the florets. Those on the outside - the ray florets often have large petal-like structures but they do not have have any reproductive organs. The florets towards the centre have stamens and ovaries, and their petals are fused into a small tube-like structure. It is the inner florets that need insect visitors for pollination. The UV patterning of the entire head of florets is not dissimilar to a darts board - with concentric circles - inviting insect visitors in. The difference between what we see and that seen by an insect is shown here.
Sunflowers make an impact (on us and their insect pollinators) through sheer size and the colours that they reflect, but the humble buttercup is not without a claim to fame. Its small but bright yellow flowers are quite special. The petals have a characteristic sheen that adds to the intensity and vibrancy of the yellow flower.
The cells of the top surface of the petals - the epidermis, are filled with yellow pigment. This pigment reflects the yellow wavelengths of sunlight but allows other wavelengths of light to pass through. Below the very smooth and regular cells of the epidermis sits a layer of cells filled with starch grains. Between the epidermal cells and the 'starch filled cells' there are numerous but small air spaces - creating effectively a thin layer of air. This "air layer” and the epidermis work together, creating a "film” (bit like oil on water) that is a strong reflector of light.
Light which passes through into the starch-filled cells is bounced around and some will pass out through the pigment layer - contributing to the deep, saturated yellow colour, and the characteristic shine of the buttercup petals. So marked is this intense mirror-like yellow shine of the petals that it has given rise to the children's 'game' of holding a buttercup under the chin: the yellow reflection onto the chin is said to mean that the person likes butter.