Early Ford motorcars were advertised for sale ‘in any colour as long as it is black’. With eyelashes you are allowed a bit more variety, if you want it.
Fully coloured eyelash extensions
You can find lash extensions in almost any colour you want, sometimes with glitter. You can have the entire set of eyelash extensions done in that colour. However, there is a reason why black is the most suitable colour for most purposes, most of the time. It gives depth to the eyelashes. Full colour sets of eyelashes can lack the necessary appearance of depth. But that can be remedied by using black lashes as a framework and then filling in with coloured lashes.
A spectrum of colours
You could start with one colour at the inner edge of the eyelid, move to another colour for the middle or the eye over the iris and pupil, and then have black lashes for the outer edge. You could emphasise your eye colour by having extensions in the same colour as your iris above the iris and pupil.
Using colour only for accents
Colour lashes can be used more sparingly, either as a change at the outer edge of the eyelid, or scattered among the basically black eyelash line. They work best when placed on the lower layer of lashes, which will make them more visible when the eyes are open.