Paul Moir has nailed it. Grey welds are from the metal being too hot when they leave the inert environment.

You should be able to easily eliminate the dull grey at least at the end of your weld, just by increasing your post-flow and/or by not pulling away from torch from the weld area so fast. Keep the gas flowing as long as it takes for the metal to cool down.

As for getting the whole weld bead to not be dull grey, you might need to develop better torch and filler feeding coordination. Using the right torch angle is also important. Specifically, try to use less torch angle (closer to perpendicular with the surface your welding) to push the inert gas backwards over the hot metal. If you angle it too far, the inert gas won't get pushed backwards enough. Another fact is that Argon is heavier than atmosphere and sinks, so you may get better results in some positions than others. (e.g., flat welding easier than vertical welding.)

Check out the video in this thread if you haven't already, it shows .065" stainless exhaust tubing being welded with great technique and results:
http://www.everlastgenerators.com/fo...-Welding-Video