From what I've read, if they dip it they'll know it's not ATF. Something about the specific chemical makeup of the dye, not just the color.
I don't know if it's true or not, but it's worth knowing so they don't "catch you lying".
That being said... I've been driving for almost 15 years commercially and have never had my tanks dipped or even looked at....
Except for one specific time. The DEA pulled me over in either Illinois or Indiana, I don't remember which one it was. I was driving Tank home from Texas, hardly any sleep and an upset stomach due to Sonic. My temporary tag had folded itself over and I had apparently pulled into a rest stop just as they pulled up behind me. Drank some pepto bismol and left again so they said it was suspicious that I pulled in and never got out of the car (completely understandable). So here I'm sitting on the side of the road talking with them while the one officer searches my truck. I've got nothing to hide and ZERO desire to give them an excuse to be difficult.... Didn't find anything in the cab so he put a borescope in the tanks to look for hidden drugs there. He let me watch which was pretty cool actually.
I was EXTREMELY glad there was nothing to be found. That would have been a REAL mess!