Interesting question! It seems there are a couple factors that can give certain strains a purple hue. In some cases, strains that are grown outdoors and in the cold will turn purple. There are also compounds (anthocyanin) that result in pigmentation depending on pH (acidic-red, neutral-purple, and blue-basic). One popular strain, Purple Kush, out of Oakland, CA, is an indica known to be helpful for sleeplessness.
However, most patients experience fewer dreams, and tend to not remember them as well, when they take cannabis before bed. It is thought that cannabis decreases REM sleep, which is the period of sleep associated with dreaming. More often, regular users of cannabis, upon quitting, begin noticing more vivid dreams!