The Forcecage spell states:
A creature inside the cage can't leave it by nonmagical means.
And its box mode states:
A prison in the shape of a box can be up to 10 feet on a side, creating a solid barrier that prevents any matter from passing through it
The Wall of Force spell states:
Nothing can physically pass through the wall.
The description of Nolzur's Marvelous Pigments states:
When you complete the painting, the object or terrain feature depicted becomes a real, nonmagical object. Thus, painting a door on a wall creates an actual door that can be opened to whatever is beyond. Painting a pit on a floor creates a real pit, and its depth counts against the total area of objects you create.
Can Marvelous Pigments be used to paint a door or hole to go through a Wall of Force or a Forcecage?
Best Answer
As you've identified, the rules are ambiguous here. It seems to me that the pigments operate by transforming the wall behind them into a nonmagical doorway, and that this transformation is magical and would apply to a wall of force just as effectively as a nonmagical wall.
But you might also argue that the wall of force is a nonphysical object, or that it regrows into the created doorframe, or simply that the doorframe's status as nonmagical means the action just doesn't work.
(It's also not totally clear if you can apply paints on a Wall of Force. Wouldn't the paints slide off?)
At any rate, when a rules interaction is ambiguous in D&D 5e, the solution is to ask your DM to make a ruling. We're not your DM, so we can't make that ruling for you. Good luck with it.