If I understand correctly, it sounds like you're using a Comcast box that has red, white, and yellow ports for audio/video,no red/blue/green ports, and you're using red/blue/green cables to connect the Comcast box and the Slingbox. If that's the case, you can still get a proper video signal, but you'll need to hook up the cables in a specific way:
Green cable --> Yellow port
Blue cable --> White port
Red cable --> Red port
The key is to do this on both the cable box AND the Slingbox. You won't be using the red/blue/green ports on the Slingbox.
Once you've done that, follow the process in this article to reconfigure the Slingbox (making sure you select the proper input for video):
Hope this helps!
Sling Media Tech Support Team