Help:VE39
From TrainzOnline
"The texture texture-filename is a uniform color."
This message indicates that the entirety of the specified texture file is the same color.
This is an error for content above trainz-build 3.5, and a warning at or below 3.5.
This message is suppressed for any texture which is smaller than 64 texels in area.
This condition is an error because the asset in question is wasting disk space, loading performance, video memory, and runtime performance for no benefit.
The correct fix is typically to reduce the texture to an 8x8 square. Since the texture contains no distinguishing features, this will not affect the visual result.