Right now my favorite color is Roscolux 383.   I've been using it all the time now.  It's a very deep blue with a hint of magenta that works great in cyc lights or as a wash for artsy-fartsy nighttime effects.  David Bell, the designer for Community Theatre of Greensboro, turned me on to this particular color during a Greensboro Ballet production he did here. 

Edited by Mr. Lowell - 3/16/10 at 10:40am

My favorite color to use is R87. I like it not for the color it produces, but the effect it has on the audience. It makes anyone in that light seem closer to the audience. If you are lighting a whole bunch of people onstage, and you want to make one of them suddenly pop, hit them with this color, and you will accomplish the task without needing to ramp up intensity.

Regarding all the rest, it all depends on what is happening onstage at that moment, the mood I am trying to create, the colors of the set and costumes, and how much movement there is. 

Stage pictures are created with a mixture of more than one color.  In order to create the best effect with a saturated blue, you need to make it stand out by providing a contrasting color elsewhere onstage, by filling shadows, or by shifting the color mixture. I think the biggest challenge a lighting designer faces is keeping the look fresh. Put someone in a saturated color, and within 30 seconds or a minute, everyone's brain adjusts to the color, and it loses its pop.