I don't remember the colors really well. I seem to think there's bright orange, or maybe yellow lines, gray lines, and blue lines. The VG is right - the blue lines are delayed routes (TEs or RBs). The rest all appear the same, as far as I've noticed.
Here's my guess: There's usually (maybe even always) three orange/yellow routes. I think that those are the "main" receivers. The game thinks those are the routes you should check first. The gray routes are the rest of the routes.
It may also be to help clear the confusion. With all the yellow/orange routes, it may be too hard to see who is who. A gray line may clear it up.
Your first read is whoever you want it to be. The way I play is when I pick a play, I can only really remember the route of one or two players. My first read is that player. First, I check blitzes as I'm fading back. Then check that first receiver. If it doesn't look good for him, I do a quick check for other receivers, and watch out for any d-linemen who have escaped my blockers. Last, I look for a running route.
Also, the way I've found it best is to pick a formation. The one I picked first is the 5-wide receivers / no shotgun formation. Learn 6-10 plays pretty well. Like, if they're blitzing, each play has a pretty good route to throw in a hurry. If they're in a zone, this guy should be open. If they're man-to-man, this guy should be open. After a while of getting those 6-10 plays, move on to another formation. (Another one I like has shotgun snap, 4 WRs - 3 bunched on right, 1 on left, and a RB on the left side.)
That's my advice.
