Well, here's a run down of the meanings of each signal.
http://broadway.pennsyrr.com/Rail/Signal/aspects_us_cl.html
I'd use either the AAR rules or NORAC rules unless you know specifically the road you are working with.
A few of the obsolete signals have disappeared from this site, but it is probably the best resource. In a nutshell, U.S. signals are much more concerned about ensuring that trains approach following signals at an appropriate speed rather than giving the train crew specific information about if, whether or exactly where they are diverging:
Medium speed = 30 mph
Limited speed = 45 mph for passenger trains, 40 mph for freight in most places
Slow speed = 15 mph
Restricted speed = Prepare to stop within one-half the range of vision-short of a train, obstruction or switch improperly lined. Be on the lookout for a broken rail. Speed must not exceed 20MPH outside of interlocking limits or 15 MPH within interlocking limits. This speed applies to the entire movement.
On an interlocking (home) signal, with respect to a single-headed signal or the top head on a multi-headed signal, a light other than red on the top head (and the lights on the lower heads are red) implies a straight-ahead route and a clear signal allows for maximum authorized speed. A green or a yellow on a lower head implies some sort of diverging route (or converging, either way, you are not staying on the same track.) On a two-headed signal, or the middle head of a three-headed signal, Medium Clear means a diverging route through the interlocking at Medium speed (30 mph; or limited speed when flashing); while a green or yellow on the third head (with the above two heads red) on a 3-headed signal imply slow-speed (15 MPH) operations through the interlocking.
The exception is the restricting signal, which is red over steady yellow, or red over red over steady yellow. "Restricting" is kind of a catch-all. It can be displayed when exiting signalled territory onto unsignalled. It can also be used (called a call-on) to display a track known to be fouled or occupied. But passage of it requires the engineer to travel slow enough to be able to stop short of an obstruction - in effect, it absolves the signal system of maintaining separation and places that responsibility exclusively in the hands of the train crew.
On distant (approach to a home) signals, a yellow over green (or yellow over green over red, both called Approach Medium; if the green is flashing, it's an Approach Limited) warn the engineer in advance to approach the interlocking at medium speed, generally preparing to take a diverging route. On some 3-block signalled systems, Approach Medium is used to warn that the next signal will be an Approach (which implies a mandatory slow-down to 30 mph). Other railroads use the Advance Approach signal for the same purpose which is a little less ambiguous. If two interlocking are closely-spaced back-to-back, then an approach-type aspect (e.g. Approach Medium) may be given on the interlocking signal. This means that, while no diverging movement may be expected at this interlocking, one may occur at the next one or even at both. If three interlockings are back to back, then the diverging movement may happen at either the second or third interlocking. Some railroads use combination signals e.g. Medium Approach Medium, which are used at back-to-back interlockings where a diverging movement will happen at two or more of the interlockings.
Oh, I should mention signal permissiveness. Simply put, most signals apart from interlocking signals are permissive in that even the most restrictive aspect can be passed by a train. These are almost always automatic signals (block or distant signals.) A red (or red over red, or red over red over red) mean that the following block is known to be fouled in some way, but a train merely has to stop and then proceed ahead very slowly watching for broken rail, fouling, other trains, etc.) Interlocking signals are NOT permissive since this potentially allows for trains to literally cross right into the paths of other, properly-cleared trains. Permissive signals usually have a number plate.
I don't know how well I've explained it, but it is probably easier than it seems at first glance. Does this help?
P.S. Here's another good source. These use somewhat more standard signalling aspects:
http://www.customsignals.com/signaling-aspects.php
