Yep, so long as your footage is set up for this, even halfway, this is a snap.
Create a duplicate of your footage, so that you have the same video on Track 1 and Track 2.
Add a Chromakey on Track 2. Use this to key out the colour you want to keep. In the original Sin City, when they were filming and knew they wanted the colour to stay in, they would make sure it was a "key-friendly" blue or green.
Now add a Tritone filter to Track 2. This will create the effect on everything but what you keyed out. You can now use your artistic bone to change the sepia colour to something you'd like. You should put a Hue-Sat-Lightness filter on Track 1 to change your green blood back to red. Then do any further colour correction changes you have to make.
Voila! One Sin City effect. Simple. (You can do this with a other filters and the Pixel Chooser, but seperating them out onto two tracks makes it easier to do the cosmetic changes to get your look right)
Curious Turtle Professional Video
Training | Editing |Support