Larry is correct in that you can run all of those schemes together and be prototypical. You can check the timeline on the Bluford Shops website to see if there are any Union Pacific references there that would help.
True; I've seen the odd photo of yellow-painted cars taken within the last few years.
But there are
minimum cut-off dates for certain things if you want to really be accurate. If UP dropped the "Railroad" from the herald in 1970 like Craig notes, and our friend asking the question is modelling the 1960s, then that would be anachronistic/inaccurate. If he's modelling the present, then he could include a car in the yellow, but it would be heavily weathered as it's in original paint from 40 years ago.
If you actually model a particular year, you can even try to get your hands on an equipment register and look up particular car series to verify whether any were in service in that time, but that doesn't tell you anything about what they look like.