This property is used to change the color and transparency of an effect over time. The values are in terms of "time/r/g/b/a", each separated by a "|". The TimeToLive property says how long the object will actually live. See the Percentage Series page for an example. The r/g/b is the color of the object, with 255/255/255 white, 255/0/0 red, and 0/0/0 black. The alpha is how opaque the effect object is, a value from 255 (fully opaque) to 0 (fully transparent). The color and alpha is then linearly interpolated between the colors and alphas set for the given times.
Source and more details
According to one test, the minimum transparency before a texture becomes fully transparent is 9, which means 0 through 8 is effectively the same. As of this writing, it has not been tested on other properties with a transparency setting, but I expect it may also be the case for all of them.