Nope, it doesn't matter what colors Platies get pregnant by. It will just effect how the fry look. However, this is off topic, but you really should have 2-3 females per male, so one female doesn't get totally harrassed and stressed out by the male.
Anyhoo, you can look for a gravid spot, which is a dark spot near the anal fin on the female. In some colors you can't see it, so if you can't, don't worry. If you can, see if it's big and dark. You might even be able to see little parts of fry, like the eyes. Though I would certainly say she sounds pregnant.
You can feed her 2 times a day, or maybe even 3. Just make sure to alternate her foods, like not all High-protein foods. And really, you can't make them so fat they have troubles delivering. However, I will say, if the male only bugs her, she can has stress problems and then birthing issues. Any way you could seperate her into a seperate birthing tank? (Preferably not a trap or net). Then, once she gives birth this could also double as a fry tank.
If you want to set-up that tank, here's some basics. I might think of more later and add on.
Temperature is good anywhere from 75-78 F or so. I think a sponge filter is best for a fry tank, so that they can't get sucked up. It's worked well for me in the past, so I would recommend it. Otherwise, you could use a HOB, or other filter and just cover the intake with a breeder net, or Pantyhose, or something similar so they don't get sucked up.
Ornaments can be anything you want...Plants are good, as are rocks, wood and general places for them to hide. Thick plants seem to be a favorite with mine. They can disappear into there, and fool their brothers and sisters as to where they are! Sand is the best substrate. With gravel, they can sometimes get stuck in between the pieces and get crushed. Bare bottom also works, and some find it easier to clean.
Fry need good water quality, and they are sensitive so lots of water changes! Daily ones are okay, once every 2 days is okay too. Just make sure the tank is always clean. You can do water changes using a airline tubing siphon you make, or a turkey baster so you don't suck up the little babies. Acclimation to the tank is impotant too. Take a loooong time...and they'll be better off.
Also, fry need to be fed often! 3 times a day at least, more if you can. When they are first born, Tetramin Fry Food or Hikari First Bites are best for them. Baby Brine Shrimp is good suppliment to this, or you can just feed that but it's best to vary. Once they grow up a bit, you can add in finely crushed flakes and Freeze-dried, frozen, or Live foods, such as Bloodworms, Brine Shrimp, Tubifex Worms..etc. However, don't feed them just this. These are all high protein foods, and they can cause them to be constipated or get bloated. Fry are sensitive. You can also try small pieces of blanched veggies, like cucumber, zucchini, peas...etc. As they get older, you can feed them the same foods, but bigger. And eventually you can cut down to however many times a day you usually feed your fish.
Just keep in mind, Platy fry can breed at 3 months, so it's best to seperate girls and boys as soon as you can to prevent brother/sister inbreeding. If it accidently happens, for the first few generations, they should be fine, but as it gets further down the line they will have deformities, and not be as strong. So seperate as soon as you can.
Oh yes, and Platies are pregnant for about a month.
Best of luck, and sorry if this is too much info!
, and then at night...they become lovers again 
lol.
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