Slitherine

Additional details on the Significance of shading in area in regards to supply: red/blue, blue/red, white.
such as tracing of supply, replacements, movement.

Shading of the red or blue indicates distance from the nearest supply point. Paler it is, the further away you are with white being out of supply, darkest being the point of supply on the map edge.

Cross hatched colouring indicates both factions in that area are in supply.

Supply shading does not reference replacements. Reinforcement and rebuilds are indicated separately.

There's mention of it in the tutorial though it should also of been added to the rules.

It is at least mentioned in “Interface Hints” in Help.