I'm a closet rambler fan
My dad had a wagon just like that when I was a kid, and I spent many miles in the back seat.
Years ago one of the paint manufactures advertised an online program (like your asking about) in all the car magazines, but I never saw it actually come out, and if so, not for very long.
GIMP is nice and cheap $$$ but it's kind of hard to figure out, not for a novice.
If you could find an old version of Photoshop for sale, it's actually pretty straight forward to replace a color.
I have a ten year old version and it still runs under Windows 7 just fine.
If you post a picture of the color you are interest in, I can try to match it for you.