You can control the Color for the two states of Geometry.

•Geometry Under Defined - we usually describe this color as Blue in this help

•Geometry Fully Defined. - we usually define this color as Black in this help

... in Application-Settings > Graphics tab > Display Colors

Other Colors

•Reference Geometry is Orange

A Gray dimension is also:

•in a Design-Set

•the length of the Line between the two gears of a Gear-Pair

•the radii of a Pulley

•a Controlling-Dimension that controls the length of a Motion-Path.