This is in:

MC 1.12.2 - Skyblock 4

When you make a bonsai hopper plot (the tier 2), it has no color, but once placed upon a chest it becomes "light gray"

This isn't much of a bug, but more so an inventory/storage stacking issue.

Light gray is the default color for bonsai pots.
There are no uncolored pots.
I might not be understanding what your issue is exactly.