After taking several road trips to different noder gatherings, I started to pay attention to the colors of highway signs. Subconsciously, we know what the colors mean, but I never really paid close attention to how they are used. The following is the results of what I've figured out.
Guide Signs (Where you are, where you're going, and how to get there)
Guide signs are the most common of highway signage. They're green with white lettering, and are usually mounted on overhead structures that span the entire width of the road. These signs are used to direct the driver on how to reach their destination, and typically give city distances, exit road information, and lane information.
Examples of this type of sign can be found nearly everywhere. The city distance signs are normally found posted on the right side of the road, and will list two or three cities along with your current distance from those cities. Exit information signs will usually say something like "MAIN STREET, EXIT ½ MILE", while the exit signs themselves usually just say "EXIT" along with the exit number, and an arrow pointing to the exit ramp. Lane information signs typically show you what lanes you need to travel for particular directions. For example, if two merged interstates are about to diverge, these signs will tell you what lanes to use to continue on your desired interstate.
Guide signs also include the little mile markers you see on the side of the road (thin, vertical rectangles containing a number), interstate identifiers (a blue shield with a red crown, and white letters that say "Interstate" and the interstate number), national and state routes, as well as bridge and highway identifiers.
Regulatory Signs (What to do, and what not to do)
Another very common variety of sign is the regulatory sign, used to inform the driver of the rules for the road enforced by law. The most famous example of this particular variety of regulatory sign (and probably the most widely disregarded) is the speed limit posting. This can include both minimum and maximum speed for the defined area. Other examples include "Railroad Crossing", "Right lane MUST turn Right", and "Keep right".
Many regulatory signs can also contain the universally known "NO" symbol (a red circle with a slash through it) to tell you what you're not allowed to do. Examples are "No U-Turn" (symbolized by an upside-down U-arrow overlayed with the red "NO" symbol"), and "No Bicycles" (a picture of a bicycle overlayed with the same "NO" symbol).
Some regulatory signs are white with red text, and include "No parking any time" with arrows directing on which side of the street parking is prohibited, as well as "No parking" signs with speficied times. Regulatory signs can even be colored red with white text, the most widely known example being the "STOP" sign. These red signs also include "Wrong Way" and "Do not enter". On a trip to Ohio, I noticed that the red variety is also used for information regarding what kind of explosives are and are not permitted through tunnels.*
Warning Signs (What the roads are doing, and what might jump out at you)
Warning signs are usually less known for containing text, and are more likely to contain basic pictures or shapes. These signs are colored with a yellow background, and use black text and/or pictures. You'll usually catch these signs in the shape of a diamond or rectangle.
Examples include "Pedestrian Crossing" (symbolized by a picture in the shape of a person walking), "Deer Crossing" (a picture of a dear jumping), and "Right/Left Curve Ahead" (an up-arrow curving to the left or right). Warning signs also include bridge markers, which are usually vertical rectangles with diagonal stripes that alternate between black and yellow, or sharp curve markers which are also vertical rectangles, but have a large black angle arrow pointing in the direction of the curve.
Warning signs are also commonly appended to the bottom of Guide signs to signify an "EXIT ONLY" lane.
Service Signs (Where to get what you need)
Depending on what exit you're near, these signs can either be very useful or completely pointless. Service signs are used to inform the driver of what services are available to motorists in the general vicinity. These signs are colored blue with white text, and can direct drivers to rest areas, fuel stations, restaurants, hospitals, and lodging areas. Upon reaching the exit, these signs will also tell you the direction and distance to reach each of the posted services. Some of the more useless service signs I've seen are the ones that give information for a particular exit (food, lodging, etc.), but don't list anything on the sign. It will just say "FOOD" along with the exit number, and the rest of the sign will be completely blank.
Service signs also direct drivers on evacuation routes for hurricanes and the like.
Recreational and Cultural Interest Signs (Where to go for something to do)
These signs are a lot less common than the rest of the signs, and serve to direct drivers to points of interest that happen to be nearby. Colored with white text on a brown background, these particular signs can point you to state and national parks, public recreation areas, as well as the services available in these areas.
Construction Signs (What's broken, and how to get around it)
Construction signs are the most temporary of highway signs (in some areas anyway), and inform drivers of upcoming construction areas and work zones on the highway and/or surrounding areas. Black text or pictures on an orange background are used for this particular type of sign, and the color orange is used pretty much universally throughout all construction areas. This can be noted by construction tape, worker vests, traffic cones, and those big digital signs that tell you to move over. Construction signs can also tell drivers the time of day construction activity is taking place, and the names of roads to use for detours.
Examples include "Construction crew ahead" (symbolized by a person with a shovel buried into a pile of something), "Road Construction Ahead", "Detour Ahead", and "Men at Work". Detour signs also typically have black text on an orange background with a thick black border.
* Jurph informs me that these resemble the schemes of HazMat Placards, and can come in a variety of colors.