Good points...Thank you!
I've found that I'm still learning the techniques with paints, and for now, have better success with chalks.
I've found my best colors to weather rolling stock are yellow and white. I know that might sound crazy, but it seems like each color looks quite a bit different than what you'd expect depending on the base color of the car you're weathering. Yellow and white seem to look great on just about everything. They dull down boxcar roofs, make rust on black things look well rusty'r, highlights things well, brushed on the tops of trees it lightens the green and adds some variation to the foilage. I like a rust color for the couplers and wheel trucks, browns on anywhere someone's hand or foot would traverse, and every rooftop get some white/yellow to sunbleach. Also, the white makes any boxcar's white lettering look older and weathered. I am still very much learning the art of weathering as well as the science behind it. Here's some of the pics of three identical cars, one weathered - one with no weathering - one lightly weathered. White, yellow, rust, brown, black were the only colors used and blended together.
(Two of them will get renumbered soon.)