Adding colours to existing plain sweater pattern, best way to visualise/organise?

Probably a slightly stupid question, I've been googling it but only getting stuff on how to change colours while crocheting.

I've got a sweater pattern I want to do, but the pattern calls for a single colour the whole way. I'm a nerd so I want to to colour it to look like bird markings (an eider duck to be precise). I'm wondering what the best way to go about working out which stitches should be which colours to get the effect I want!

Any help greatly appreciated!