It involves both more painting than you'd think, and less painting than you'd think.
With magic, of course!
We used a colourful stone to bring light to the garden earlier, so may be able to do that again.
The stone brought light to the paintings in the gallery; if we could put the statue into one of the paintings, that may help to colour it in.
Remember, the gallery is part of the fairy world, and as such it's... weird. We needed to use the crystals to access a corridor the upper level.
That implies that the crystals are able to shift the paintings from 2D to 3D at will.
If the statue, a 3D object, was turned 2D, it would likely absorb the colours of the painting around it.
When we originally entered the gallery, we took the colourful stone from upstairs and put it downstairs.
Try putting it back upstairs.
Now you can put the statue on to the colourful 3D stairs; this will ensure it absorbs the colours when the painting is made 2D.
Grab the crystal from the nearby fixture. Note that this makes the corridor leading downstairs disappear, too.
You can turn the painting 3D again to retrieve the statue, but it seems we're missing the colour orange.