The flatbeds around my area all have painted sides on them. Most are red. The frames are usually red, white, or repainted black. I personally like the white frame and red sides. However, I think natural sides if stained a complementary color to the truck, also look beautiful for show vehicles. My last $0.02 is that I like the sides to be the same hight. Don't care much for the taper look. Just my opinion.

1949 1.5 Ton 4400, 1969 c10, 1989 c1500, 1990 K2500, 2010 C1500 Crew