Most any competant powder coater can make uniform colors like that. The largest trick is spacing the tube off the ground wires so you don't get a distortion pattern around them. On large parts, that is less important so some people just don't do it right.
I use foil to fill the tube except the last 1/4" of each end and run my ground wires through that. Depending on the kit, it is sometimes necessary to drill the powder out of that last section, but not always.