I'm new to this forum, been doing HO for 40 years off and on, and I'm working down my 40-year inventory of undecorated freight cars. Right now I'm looking at two hanging display cases with 45 cars I've decorated over the years. And I like the advice to use any paint I can get to match a picture. The comment from Lehigh74 got me to look at Krylon rattle cans, and I think such will do the color job based on photos I scared up of what looks to be the car that the Herald King decals emulate. I've been using that paint matching reasoning on my gaggle of WWII display 1/72 and 1/48 airplanes. Time passes, colors change, images can be distorted (I like the permanence of Kodachrome, but no telling what medium was made to take a picture or the lighting of the scene pictured).Actually, as you get more experience in this hobby, you'll realize that paint colors aren't standard. What the PRR calls "Tuscan Red" may look like burgundy to many.
Your best bet is to find a picture of the color you want, and try to match an existing color. Personally, I would use an airbrush (with Vallejo Acrylics paints) to paint a loco, but a rattle can will work.