The diagram on the right compares the range of bumblebee vision and human vision.
The centre line shows the wavelength is in nanometres, and the colours are written below the line. So humans can see wavelengths from around 400
(blue) to as far as 800 (red) nanometres, whereas bumblebees can see from as
low as 300 (ultra violet) but only up as far as 700 (orange) nanometres.
The three basic colours seen by humans are red, green and blue, and by bumblebees they are green, blue and ultra violet. Bumblebees cannot see red, but they do visit red flowers. This is because as
well as being able to smell the nectar and any other attractive odour that the
flower may emit, many flowers, not only those we see as red, have ultra violet
patterns on the petals that are visible to bumblebees and some other insects,
but invisible to us. These patterns guide the insects towards the nectaries, and are called nectar guides.