Well, except for Santa Fe red and silver Warbonnet F3s, and the Blue Comet, I have not been seduced by color scheme. I don't do one railway, but three: Union Pacific, Santa Fe, and Rio Grande. The unifying theme being that all three ran through the southern Colorado/northern New Mexico region where I grew up. I think it is fun to focus on a region or RR and model that. I imagine most of us pick the area/RR we saw growing up . . .
But I buy locomotives that I like regardless of roadname, trying to get UP, SF or D&RG when I can, but buying what I like regardless, because they are noteworthy (NYC Hudson, Blue Comet) or cool (UP Big Boy, Challenger) or I just like them (French Chapelon 4-8-2).
A few of the very famous or pretty ones (Hudson, Chapelon, Blue Comet) I leave in their native livery, but most non Up or non-ATSF locos, I repaint.
Steamers I repaint as either Union Pacific or ATSF: I repainted the Southern Crescent in black as a UP (I know, sacrilege, but I did it) and will paint the N&W J I have one order in UP's grey and black scheme (a la Challengers) when it arrives.
Any diesesl I repaint become UP only - I can do the UP armour yellow and gray with red band well and re-letter with no problem. I don't repaint in ATSF warbonnet, because I can not do the curves/bands between the red and silver accurately and well enough to look good.