With the introduction of the Smart Tool in Media Composer 5, there are more things to be aware of. Here's an overview of what the Trim Rollers and the colors indicate.
Single Roller: When you have just one roller, at a transition on a track, it means that you are trimming just one side of the transition on that track. In other words, you are only trimming just the A-side or just the B-side shot at the transition on that track (see the previous Trimming terminology section).
Double Rollers: When there are two rollers side by side at a transition it means that you are affecting both shots on either side of the transition at the same time. In other words, you are trimming both the A-side and the B-side, adding material to one shot (extending it) while simultaneously removing the exact same amount of material from the other (shortening it).
Yellow: Yellow tools indicate that they have the power to change duration of tracks. For example, Splice adds material to the track, which lengthens it, and pushes other segments down the Timeline to the right. Extract removes material from the track, which shortens it, and the remaining material on the track then moves to the left. When trimming with a single roller which is yellow, Avid refers to this as a Ripple Trim. For those that have used the software prior to the Smart Tool, the behavior of yellow, single roller trimming is identical to single roller trimming in the past. When you're adding material to a segment (a shot) with the yellow Ripple Trim, shots to the right-hand side of it are pushed down the timeline to the right. When you're removing material from a segment, shots to the right of it are pulled up the Timeline to the left.
Red: Red tools indicate that they do not change the duration of tracks. For example, Overwrite places new material into the Timeline on top of whatever was there. It's not adding additional material, it's simply replacing it. So, the duration of the track remains the same. Lift removes material from the Timeline, and it maintains the duration of that track by leaving behind an equal amount of Filler. When trimming with a single roller which is red, Avid refers to this as an Overwrite Trim. I quibble with this name a bit as I think it should be called Overwrite-Lift Trim. When you're adding material to a segment (a shot) with the red Overwrite Trim, the adjacent segment that's in the path of the trim is actually overwritten. In other words, material from the segment you're extending (adding to) is replacing the adjacent shot's material. Note that when you're extending a segment with the red Trim Roller, the result is identical to using Double Roller Trim. On the other hand, when you're removing material from a segment (shortening it), the red Trim Roller leaves Filler behind in order to maintain the duration of that track (just like when you perform a Lift).
Purple: Whenever you have two Trim Rollers on a track, they will be purple. Purple rollers will appear when you are in Double Roller Trim, Slipping, and Sliding.