"A light blue jacket."
By context mostly, or by comparisons to things with known colours (like sky), or you just don’t – if you don’t need to.
In general languages have different classifications of colours. English has only one red but Gaelic has dearg (the saturated red of blood or red ink) and ruadh (the brownish colour of natural red hair, rust, copper), Polish has similar distinction: czerwony vs rudy.
It feels absurd to a Polish speaker to use the same single word for the colour of blood and the colour of redheads or rust – how can English cope without this distinction?! They’re obviously not the same colour. And yet English manages well, lots of people speak it everyday without problems and no-one feels the need to keep the two colours apart.
Old Norse blár (cognate with English blue) could mean both blue and black (hence blámaðr ‘black person’).
Irish glas can mean both natural green (colour of green grass) or gray (like fur of gray animals), and uaine for more artificial shades of green (like a green-painted wall), and then there’s liath for light-gray colour of human gray hair…
Polish has three colours for blue: niebieski (the colour of darker sky and the general name for blue), błękitny (bright sky, light blue), modry (now archaic, much deeper blue).
I haven’t found a similar image for Scottish Gaelic, but this Colours in Irish image might be helpful since it’s a closely related language (but bear in mind that Scottish Gaelic meanings might differ in details, and there are spelling differences: rua instead of ruadh, buí instead of buidhe, etc.).