If the platforms are short, then it really doesn't matter where you stand or what color the cars are painted. The train requires two (or more) spots because not all of the cars where passengers will be boarding or alighting can fit with the doors on the platform. Usually (except for trains like the Builder or Lake Shore with sleepers at both ends) the double spots are one spot for sleepers, one for coaches. So, in your scenario, all the red cars will still spot/load at once, then all the blue cars. There's nothing a passenger (or the crew, for that matter) can do to change that, even if they knew exactly which car they were boarding.
I doubt you'd be able to see a red or blue car coming from a mile away (certainly not literally, and realistically, not even figuratively either). Unless there's a curve just beyond the platform, one really can't see what cars are on the train until it gets right up on the platform. Even if you could, you wouldn't know where the door was going to be because it's up to the conductor and engineer to spot the train.
Again, from my experience, the train usually pulls up and the first section is boarded. Then, it pulls forward a little more and boards the second spot. So, you know that if your cars are hanging off the back of the platform, it must not be for you.
Perhaps my experiences are unusual, but I've seen the following happen many times on the CL:
1) The train pulls up to the platform to board the forward sleepers first. Since the sleepers are towards the front, the sleeper is towards the left of the platform. All the passengers have been milling about in the middle of the platform.
2) The SCA of ONE sleeping car opens his or her door. Since it's the only open door on the train, about half the people on the platform head for the open door. The SCA tries to shout that this is for SLEEPERS only, and that coach pax will board in a minute. He or she often calls out my name, & the name of another sleeper pax if there happens to be one. People try to board anyway, and mill about in confusion.
3) I board the car. The train pulls forward just enough to get the coach cars onto the far right side of the platform.
4) Half the passengers were already headed that way, but the confused Amtrak newbies are still way at the left end of the platform. We wait while they walk all the way to the other end and board the coach cars at the rear.
I think some of the confusion would be cleared up by making the train cars more clearly marked. It doesn't have to be color coded or confusing - a big "COACH CAR" or "SLEEPING CAR" written across the middle - in ANY color - would be helpful.
Most people don't know their train number, their car number, or even the name of the train that they're supposed to be on. All they know is the departure time and the city they're headed to, and they know what "class" they're in. Airports work pretty much the same way - most people know that they're on the 7:30 United flight to Chicago, and you can pretty much find it from there.
Oh, and my proportions can be off by a lot if one of the trains is late. The CL often arrives after the LSL's departure time. When that happens, the combined LSL and CL crowds both try to get on the CL, and there's much confusion and consternation by the LSL passengers who don't understand why they're not being allowed on the train. Considering that the CL uses Superliners and the LSL uses Viewliners, it shows you how little the general public knows or cares about rolling stock, consists, train numbers, or anything.
