general rule of thumb:
white smoke = head gasket issue
black smoke = piston ring/ valve issue
grey smoke = head gasket and piston ring
blue? = some radioactive shit is going on get the **** out of there

i am assuming that if the head was brought to a machine shop, then they would have inspected the valve and check for warping issues...so those are pretty much eliminated unless you went to like tim bob shop or something....also sometimes the smoke could just be coming from the muffler itself with shit inside it.....take the muff off and run a can of carb through it to see if it still smokes