BattleLore, which also uses the Command and Colors system, has a "battle back" rule. Perhaps the rule could be adapted like so:
When an Infantry or Armor unit is attacked in close combat, a player may choose to have the attacked unit Fight Back if it did not retreat and was not eliminated. Armor units, however, may not Fight Back if the attacking unit is Infantry. To Fight Back, roll the appropriate number of dice, checking for unit, terrain, and other effects. Resolve combat as usual. A unit that Fights Back may not Take Ground or Armor Overrun afterward even if the attacking unit retreats or is eliminated.
Only one round of fighting back may occur per attack order (e.g., the attacking unit can't "fight back" after having attacked, even if the target fights back). A unit may, however, fight back more than once in a turn if it is attacked by multiple units in close combat (once for each unit that attacked it in close combat).
The Fight Back tactic may neither be used against or by Aircraft. Air Checks are resolved as usual.
It seems reasonable to eliminate the restriction preventing a unit from fighting back unless it has a friendly unit adjacent to it (one example of "bold" morale in BattleLore) because close combat is much less common in Memoir 44 than the equivalent in BattleLore.
It also seems realistic. Units would not always shoot back when taking fire from the enemy (e.g., seeking to gain ground, keeping cover, pinned down, saving ammo, or orders). However, when in close combat, it becomes fight-or-die and (depending on the scale of distance represented by a hex) ammo becomes less of a question (knives, bayonets, fistfights, or picking up ammo from fallen soldiers). Tanks would have a much more difficult time fighting back when attacked by soldiers, and artillery would be difficult to aim quickly and safely enough in the duress of close combat.