From: Bexhill-on-Sea, E Sussex
Not too sure if this answers your question, but just had a look and there are three things I can see:
1. It also highlights all it's attached units in the same colour.
2. On the HQ itself, on the left hand side from top to bottom, regime symbol, a card symbol (only appears if enough command points to play a card) and then a number which is the number of command points available.
3. The green hexes on the map show units within HQ range. If a sub unit is within green it is in full HQ range, if it is in the three outer colours it is still within HQ range but doesnt get the full HQ bonus. If you select a sub unit within range and then look at the 'unit details' tab at the bottom of the screen you will see HQ power, HQ combat and HQ morale mod. Still getting my head around them them, but to the best of my knowledge (stand to be corrected), HQ power will be 100% if within 'green range' and reduced on the outer ranges (zero if totally outside) and then the combat and morale mods are to do with the HQ's stats and also the distance from the HQ.
3's the answer, thanks.