Frequency is how many swings happen per second; wavelength is the length of one swing. The two are tied by .
In empty space does not move, so raising the frequency shrinks the wavelength by the same factor. The bar in the figure is the frequency, and as it grows the crests crowd together. The speed is never touched.
Name one and the other is named with it. The same light can be called by its frequency or by its wavelength and it is the same light either way. Radio work happens to speak in frequency and optics in wavelength, and that is all the difference is.
The eye responds from roughly 380 nm to 780 nm. Longer than that is infrared, shorter is ultraviolet, and neither registers as sight.
Being invisible does not make them absent. Infrared arrives on the skin as warmth, and ultraviolet burns it. The width of the visible band is simply the range the pigments in the retina happen to react to.
Only the visible stretch of the band in the figure carries colour. It fades towards both ends because the eye loses sensitivity there.
Around 700 nm the light looks red, around 550 nm green, around 450 nm blue. Move the pointer in the figure and the place on the band and the colour of the wave below it change together.
The wave below is drawn with its length in proportion to the wavelength, which is why the crests crowd together as the colour runs from red to violet. The colour changing and the wavelength shrinking are one fact seen two ways.
Colour is also something the eye and brain construct. Two things that look equally yellow may be one wavelength at 570 nm or a mixture of red and green. Wavelength and colour match one to one only for light of a single wavelength.
Sunlight and lamplight carry the whole visible range mixed together, and the eye reads the mixture as white. There is no wavelength that is white.
Send it through a prism and it fans into a band of colour. The speed inside the glass differs slightly with wavelength, so each wavelength bends by a slightly different amount. Shorter wavelengths bend more, which puts violet furthest out.
Newton showed that gathering the separated light back together returns it to white. The prism is not adding colour; it is separating what was mixed. A rainbow works the same way, with droplets of water in the air doing the prism duty.