Plants use blue light as an indicator of light intensity.
Spectral composition of sunlight is fairly consistent during the day. Clouds can reduce the intensity by up to 90% however have little effect on the colour spectrum. When it becomes interesting is when sunlight reaches a plant. Sunlight is used as a source of energy in photosynthesis through absorption by colour sensitive pigments. Although plants use all the colours of light that fall between 400 - 750 nm, light absorption by the colour sensitive pigments is highest in the blue (400 - 500 nm) and red (600 - 700 nm) region.
It is important to understand the ecological context of plants. Competition for light between plants is high. As sunlight falls on a leaf at the top of a canopy it absorbs a lot of blue light, transmitting light without it. Therefore the spectral composition of light at the top of a canopy is vastly different to that available to plants underneath. A plant uses the presence of blue light to measure light intensity. This is what dictates a lot of physiological responses.
Plants that are exposed to blue light stay compact because it thinks it is in intense light. This can either be at the top of the canopy in a large forest or in full sun in the middle of a field. The plant focuses its energy on collecting more light and doesn't waste energy on growing out the stem. Conversely, plants that grow underneath the shade of a canopy will elongate their stem and head towards areas of blue light to optimise photosynthetic activity.
Blue light will create smaller leaves. In intense light a plant doesn't need to grow large leaves to collect a lot of light. It can keep its leaves smaller and collect all the light it needs. Again the opposite is true; a plant with little access to blue light (i.e. underneath a canopy) will expand its leaves.
When growing under artificial light we can use this information to dictate the way our plants grow. At ample we have engineered a colour spectrum that is ideal for a wide variety of indoor plants.