WC2: persist color in corner case when using the workaround

This happens if player 4 and another one choose the same color.
(They shouldn't do that to begin with, but they could)

In this case the workaround is also triggered and side 4 gets a new color.
That's fine.
Previously that would have caused side 4's color to change in each scenario,
thus it's now persisted as well.

The workaround is for the case a player chooss choose the same coklor as side 4.

data/campaigns/World_Conquest/lua/game_mechanics/color.lua

@@ -76,6 +76,9 @@ function wesnoth.wml_actions.wc2_fix_colors(cfg)
 		end
 		side.color = available_colors[color_num]
 		taken_colors[side.color] = true
+		if side.side == 4 then
+			side.variables.wc2_color = side.color
+		end
 	end
 end