@snake5 hi. I have investigated my problem a little bit and i think i'm almost done :) i disabled GL_BLEND and now colors are pretty neat, i guess there are some settings to do when enabling GL_BLENDn, atm, i just disabled it.
ideone.com/i9VmzW i'm now able to mix 3D and 3D projection, the 2D image is drawn correctly, the triangle too but after being rendered, the triangle seems to shrink... or just translate on Z axis