The battery usage screen has an indicator for "Signal strength" that, instead of being blue/black like WiFi, has many colors: green, gray, yellow. What's their meaning?
For easier reference:
I assume that you mean on the graph you get if you go to Settings -> About Phone -> Battery Use and tap the graph at the top? I have a signal strength line there, under the battery level graph that has many colours in it that's labelled "Phone Signal" on my device.
The reason it's there is that your signal strength can have a big effect on your battery life. If you have a good, strong signal from a nearby phone mast then your phone will run it's antenna in a low-power mode, whereas when it has a bad, weak signal it will turn up its transmitter into a full-power mode to help ensure that your communication gets out. As an extreme example I know that if I forget to switch my phone into Airplane Mode before putting it in the metal lockers at my sports centre, then I often come back to only a quarter or less of battery charge left.
I can't find an official reference for what the colours mean, the feature was introduced in Gingerbread but the official Gingerbread manual barely mentions the graph and that screen beyond telling you that it exists (p.48 of the 2.3.4 manual linked), from personal experience I know that Red is a bad/no signal, but other than green being a good signal, I don't know the significance of the other colours.
Some internet forum posters suggest these meanings for the signal strength graph, which seems to match up with what I see:
Red = No data signal (or possibly no signal at all)
Grey = No or minimal (2G/EDGE) data connection
Olive Green = 3G/4G connection with light data usage
Bright Green = 3G/4G heavy data usage (streaming media, etc)
Yellow = No consensus on what this means, but it only seems to show up as a very narrow sliver, and generally only when the connection seems to be changing state (eg from data network to wifi)
This seems to agree with what I see on my phone, that while wifi is on and connected to an access point (so any data should be going through wifi and not the phone network) I only have grey, olive or red in the signal strength, whilst when wifi is off I get the greens in addition.
There is also the indicator in the Android status bar that is normally green, but changes to yellow and then red when it is extremely low, the main battery usage graph follows these three colours with what looks like the same thresholds, but that one doesn't sound like what's confusing you.
Black/transparent: Power off
Red: Phone is trying to find a signal
Yellow: Signal strength 0 (min)
Dirty yellow: Signal strength 1
Even more dirty yellow: Signal strength 2
Somewhat yellowish gray: Signal strength 3
Green: Signal strength 4 (max)