I know for sure its white. Don't try it... if you do your at your own risk... but I saw my dad try this... he took a screw driver, removed the ac cap to where you recharge and check the system, he pushed down on the valve and a line of white smoke came out. I'm not sure of what it smells like its been a while.

You probably have a small leak as soon as you get into higher mph it puts more pressure on the A/C compressor probably causing it to be a major leak.

Best advice I could give you is find an expert on A/C systems. I'm pretty sure some places have coupons for free A/C check and inspection.