Very small detail I just noticed…
The color selector for Boxes doesn’t include the top section of colors used in the space.
But, when in the color picker for a card, the top section for colors used in the space does in fact include colors used in Boxes (I know you can’t tell which are cards and which are boxes, so you’ll have to trust me):
Vs when picking a color for a box, the colors used in the space section doesn’t exist: