Wavelength can be measured from any point on a wave. To avoid confusion, it is best to measure wavelength from the top of a crest to the top of the next crest, or from the bottom of a trough to the bottom of the next trough so that the measurement is of the length of a single complete oscillation.
The wavelength of an electromagnetic wave is measured in metres.
The greater the energy, the larger the frequency and the shorter (smaller) the wavelength. Given the relationship between wavelength and frequency — the higher the frequency, the shorter the wavelength — it follows that short wavelengths are more energetic than long wavelengths.
The visible spectrum is often divided into named colours, though any division is somewhat arbitrary.
Traditional colour names in English include red, orange, yellow, green, blue, and violet. But the visible spectrum is, in fact, continuous, and the human eye can distinguish many thousands of intermediary spectral colours.
Wavelengths corresponding with the colours of the visible spectrum are usually measured in nanometres. There are therefore 300 different colours between 400 nanometres (violet) and 700 nanometres (red). But if picometres are used instead, then there are 300,000 different wavelengths each of which corresponds with a different colour.
A colour produced by a single wavelength is called a pure spectral colour.
Light is rarely of a single wavelength. Light is usually a mixture of several different wavelengths.