My experience is (and I presume the laws of physics dictate) that the more open the air shutter is, the more blue and sharp the flame will be, up to the point where it will start to roar and jump off the orifice.  Closing the shutter will cause the flame to yellow and lack definition.

The problem might lie with something other than the air shutter setting.

The air shutter should be open completely unless the flame is blowing away from the mouth of the burner tube opening.

There is a big difference between orange flashes and yellow at the far ends of the flame, the periodic orange flashes are bits of dust, lint and other dirt being burned [ nothing wrong there ] the yellow-whitish flames are a sign of too little air, however almost all gas dryers have quite a bit of this type of coloration in the flame so I would consider it normal.

Dryer getting too hot? Be sure you have tested the safety and regular operating thermostats. Test the safety thermostat by running the dryer with the door completely open, the gas burner should shut off within a minute or less. Test the operating thermostats by running the dryer empty and use a probe type thermometer inserted into the exhaust duct while running the dryer empty, the burner should shut off between 140 and 180 degrees depending on the temperature setting and your particular dryer.

If you start a dryer like this empty and let it heat for a few minutes parts like the like filter will quickly get way too hot to touch, these parts heat faster than the operating thermostats that will shut down the burner in a few minutes. At this point I would dry a few big loads of clothing and see how the dyer is operating.

I would not even attempt to test the gas pressure at the dryers gas valve, you may cause some other problem or get into a dangerous situation.

To what? I would need lots more details to answer your problem. What kind of dryer for one, you can not see the flame in any modern dryer when it is operating.

Was the dryer properly converted for use on LP gas ?