Idioms use language metaphorically rather than literally. If you have 'green fingers', it means you are very good at gardening and looking after flowers and plants (the metaphorical meaning) not that your fingers are the colour green (the literal meaning).
Idioms are also fixed groups of words so you can't change the wording of an idiom. For example, you can say 'She never lifts a finger' to mean that she's very lazy but you can't say 'She never picks up a finger'.