Forerunner symbols

In the Halo Universe, there are signs above doors, on Elite armor, virtually anywhere on the halos, and in the Covenant.

According to Bungie, namely Joseph Staten, the blue and red signs mean a 'safe zone' as in whether or not the Flood are there.

Symbol Analysis
Taking a closer look at the symbols and where they were found and what has been said by Bungie we can deduce the following:  Image:CRsymbol1.png|The glowing cyan symbols are found on the control panel in the Halo control room. Image:CRsymbol2.png Image:CRsymbol3.png Image:CRsymbol4.png Image:Elitesymbol1.png|The non-glowing cyan symbol is found on the back of higher-ranking Elites. Image:Plasmasymbol.png||The yellow symbol is found on the Covenant plasma grenade. Image:Forerunner_lift_Symbols.png|It can be induced that the purple symbol means Elevator or Lift, most likely, as it is seen above doors leading to elevators. Image:Floodsymbol1.png|Above the doors where large Flood storage rooms are, two of the red symbols always appear, which backs up what Staten said about red and green symbols. Image:Floodsymbol2.png 

All the others are found in various Forerunner structures, mostly on the level, 343 Guilty Spark (Level).

On the walls in the silent cartographer are another type of symbol. It does not glow and it is not easily noticed but it is there. If you look closely at the walls you will notice lines that make patterns. One of these patterns looks very much like a human face. This would support the theory that the Forerunner are connected to humanity in some way. This is easily noticed on Silent Cartographer, but can be found in almost all Forerunner structures. Example: Coming out of the room where you unlock all the doors, look on the wall directly in front of you and you will find a human.

Cropped Symbols Analysis
Several of the symbols are variations of each other. There are a total of 16 unique symbols; the other 8 are rotations or croppings of the 16 main symbols. Of note is the fact that the cropped versions may not have been intended to be cropped. They appear on a wall or something, similar to the non-cropped version, but they are placed (as decals) in such a way that part of their texture extends past the edge of the poly that they are on, thus clipping it off. But, in sake of being thorough, they have been included as separate symbols.