to mod the paintjobs you have to find the paintjobs txds.
these are named as the name of the car with PJ's, and a number after. to mod them, open up imgtool, and find elegy1. this is the easiest to start with. extract elegy1 from imgtool, and open txdworkshop. open elegy1 with txd workshop, and extract the one PJ file. now you can edit it in photoshop or whatever you have. when youre done put the file back in elegy1, and put elegy1 back in img tool. now run the game, go to wheel arch angels with your elegy, and use paintjob 1. if you did it correctly, youre new paintjob will be on the car.
hope this helps!