I also have one of the RailKing water tenders behind a LM Challenger, and it sizes up perfectly. The tender is yellow, but it seems it would be very simple to paint it black, if someone wanted it to. There really isn't anything that even needs to be masked off. Just spray it and decal it. The silver trucks would have to be painted as well, of course. I'm pulling UP yellow passenger cars with silver trucks behind my Challenger, so the RK yellow tender with its silver trucks fits perfectly in this consist.
Not on the topic of size, but just as an aside, one thing I found remarkable is that the yellow paint on the Lionel passenger cars I'm using (these cars are some of the 15" aluminum highly detailed cars Lionel did with diecast sprung trucks, finished and painted interiors with figures, add-on roof vents and antennas, grab irons, etc. that originally came with the Anniversary set headed up by scale ALco PAs) and the yellow paint on this RK tender, match perfectly.
Yellows are hard to match anyway (there are an enormous number of different shades of yellow), and to have the shade of yellow paint from two different manufacturers match exactly is almost a one-in-a-million thing. I know of other UP yellows from MTH that are different, so it was quite amazing to see this perfect match. Almost weird.
This color match issue brings up one other point, this with the grey painted version of the RailKing UP tender, and whether the grey paint on the RK would match the grey on the LionMaster tender. If they don't match, running them next to each other may not have a very good appearance.