If you were going to paint a bookcase two colours, would you...

[concooktion]

... paint:
Outside surfaces one colour and inside the other?
Different colours for each "box" of the shelves, outside edges one of each?
Uprights one colour, horizontals the other?

Darker outside lighter inside

Depends what you are trying to do with it (and what the colours are!)

Outside one colour, inside another is more sophisticated, especially if using muted colours.

Different colours for each box would be more playful (but I'd still make the outside edges a single - probably darker - colour, to unify the look)

trying to do is a bit of an overstatement. DS has declared his bookcase should be these colours. I want it to be as long lasting as possible (and do I need to treat the wood before painting it to make it easier to remove the paint if he changes his mind in the future?).

Green outside, orange inside (or orange inside back board, if there is one and).

They are both kind of hideous though and you will struggle to paint over it in future. You would be better off painting the outside white and playing with the green and orange inside.

Those colours are kind of hideous together.....can you paint the uprights and shelves white, and then just paint the insides in the green or orange?

Those colours are awful! But I'd do green on the outside and the shelves, and orange on the inside. And then I'd take him to get his eyes tested!

they are a brighter in real life than they appear on the screen. It's his room...