In a vague sense, but not really at all. The upper and lower heads are NOT independent of each other, the specific combinations of ALL of the vertical lights make up the signal indication (meaning that is conveyed). Each vertical mast applies to one track.
However, diverging or reduced speed routings will have the greens and yellows in lower positions, and straight or unrestricted routes will have the greens and yellows in the top position. (e.g. "Clear" would be Green over Red, "Slow Clear" would be Red over Green. You'd get a slow clear if you're crossing over, but it doesn't exactly translate to "the bottom light is for the side track or crossover")*
Signal indications also depend on the next signal down the line, and there are some approach indications that combine yellows and greens on multiple heads. For example, in my Canadian rule book, Yellow over Yellow is "Approach Slow" (no restriction at this signal, but be prepared to pass the next signal at Slow speed).
*Note also that there are different rule books, so some of the names and colour combinations may vary a little bit between railroads. It's generally quite logical once you understand the pattern however.
A top Green (Green over Red) is pretty universally "Clear", and high Yellow (Yellow over Red) is pretty universally "Approach" (prepare to stop at next signal).
They include a couple of different sets of signal indication rules there for different railroads. Here's some of the indications from the older Canadian rule book, in use from the 1960s to early 1990s (a new rulebook was introduced, but all of the indications in the old rulebook still exist; there's some new ones and some have renamed but all the old meanings are still the same): http://www.wrmrc.ca/ucor/281.html
It all looks a bit complicated at first glance, but the indications themselves generally follow some very logical rules:
-The lower the (non-red) colour, the more restrictive the signal (e.g. high Green is "Clear", middle green (with 3 heads) is "Medium Clear", and bottom green is usually "Slow Clear"*
-A flashing aspect is less restrictive than one that is otherwise identical, but solidly lit
-A high yellow is an approach signal, and can be combined with other colours to indicate the speed to approach the next signal (Yellow over Red = "Approach" [stop at next signal], Yellow over Yellow = "Approach Slow", Yellow over Green = "Approach Medium")
- The addition of a lettered plate below the signal can modify its meaning.
*(I'm used to working with the Canadian rulebook. Some indications may vary if you're using a different rulebook. If you don't want to actually represent a completely prototype system, and just have something to indicate switch position or block occupancy, that's fine too and you can set up something simple and logical that makes sense; but if you do want a system that accurately represents prototypical signalling, you'll need to choose the right rulebook for your railroad. If you're freelancing, choose the one you like the best.)
I think Kalmbach also had a paperback book on railroad signalling, but I've never read it.
A note on intermediate signals and the "Stop and Proceed" indication:
Signals basically come in two types: "home" and "intermediate" signals.
Home signals are the control signals at interlockings and CTC control points. These will all be configured to display a STOP signal as the most restrictive.
Intermediate signals are distant signals and block signals located in between points. These are distinct from home signals in that they display a STOP AND PROCEED indication as the most restrictive.
Note the difference between how "Stop" and "Stop and Proceed" indications are presented in various rulebooks.
Most web sites and magazine articles about ABS signalling will tell you that "Stop and Proceed" signals have number plates, and "Stop" signals don't. While this is certainly true for a lot of American roads (especially the western ones), this is NOT a universal truth. It's certainly not true at all in the Canadian rules, and pretty much every signal up here has number plates.