If not why does the bulb go red, I have noticed in most cases it seems to happen when the corp morale is very low
It is nothing you have done.
It is to show the corp ai is inactive, or will not necessarily respond to your orders, should still defend itself where it can, ie infantry squares etc.
Causes.
Morale as you say is low.
Low cohesion.
High loses. (approx 45%).
Fatigue.
Fixes.
You should probably look at retiring/saving corps with less 50% loses, should still be able to move them out of danger...
If it is about saving troops use the retreat corp order. (only old gui at moment (press g)).
Change affections of remaining units to nearby corps is another possibility.
Try to keep good cohesion within army, don't let corps get cut off.
Good organisation within corp when building oobs is a good start, divisional level organisation is import, if you group artillery in one division they will normally stay together, ie grouped together in one place, same with cavarly.
ie: for simplicity, if you have 2 div, and you want cavalry on flanks construct your divisions with for example 2 cav, 2 batteries, 3 regiments.
When in deployment phase you can change the positions of individual units, but for them to hold the positions given you must remember the corp reserve size and answer no to modify front line. So you can place cavalry on wings, foot artillery to front, horse artillery in reserve etc...
When moving into action answer yes to modify front line, so that the ai can rotate tired and depleted troops.