A while back I posted a message asking how to make everything black and white but 1 color. It was suggested to use the BCC Correct Selected Color Filter and that worked perfect. However, is there a way to do the same thing with more than 1 color? Have everything black and white except red and blue.
The answer is yes, but you'll have to do a little compositing to complete the effect. First, duplicate the clip so that you have three copies stacked over each other. You should then use the Linear Color Key to isolate the reds on the top layer and the blues on the middle layer. You should set the key filters to composite inverted alpha on both layers. Then apply the HSL filter to the bottom layer and set the saturation to -100.