What is the difference between a yellow alert icon and a red one?

So I'm currently going for the "Foxiest of the Hounds" achievement and I need to know whether or not the "!" when it goes red means that I've caused an alarm and need to reload.

I'm confused about this because it usually goes to a yellow "?" icon but stay visible long enough it goes to a red "!" but they don't go hostile.

A couple of times I've seen brackets around the red "!" meaning they're probably calling in reinforcements or sending out a call to sound the alarm, but this doesn't always happen.

Solution 1:

The yellow marker means that the guard is suspicious - he will walk to the place he thought he has seen you. The red mark means that the guard has seen you - he will either run towards the alarm or start shooting (alerting this way other guards).

From the wiki

NPC states such as guards becoming hostile, search phases and panic do not affect achievement progress, however the same NPCs may seek out an alarm panel, therefore it would be prudent to have a means to quickly neutralize them, or try to avoid triggering those states altogether.

The most sure way to find are you still on track towards achievement is checking the "Smooth Operator" bonus that you can get at the end of the mission's step