You could try a product called Goo-Gone to dissolve the old Contact Paper adhesive. Otherwise, I suppose you will need to resort to sanding the Contact Paper off.
When the insides of the cabinets are clean and sanded smooth, they can be painted. I like to use a semi-gloss white. Easy to wipe and reflects light from the room, making it a bit easier to see what is in the cupboard.
However, if one has age-related vision problems and glare is a concern, use a clear satin or eggshell finish (whichever is one step up from a matte finish) on the bare wood, and non-stick shelf liner.

Painting the inside of the cabinets is a personal choice, really. If they're beat up, stained, have lots of holes, etc., then paint them. If they're dark inside, paint them light. Do you have pull-out shelves? Don't paint the runners, the idiot who flipped our house did that. What a pain.

Thank you both I wanted to Paint them they are very dark. I appreciate your response.