I don't think there is a need to color or camouflage the the water. You can fill a can with water that has had gas in it and it will still smell like the gas (water should be odorless). As for the color, are they really going to look at it? I don't think so. Once that spout hits their tank they will drain the can. If you leave a couple of ounces of gas in the can before filling with water, then if they do look inside they will only see the gas.
A shop I used to work at had a fork lift that ran on gas. We used to keep a 5 gal. can filled and stashed on the loading dock (which was accessible even after hours), all the employees knew about it. Several times I found the can was empty, but I knew that the fork lift didn't get it. So, I filled it again to make sure, and placed it in it's hiding spot. Sure enough, the next morning it was empty. I filled it again, this time with water, and put it back. Empty again the next morning. We also had one employee call in and said he couldn't make it in that day. He was having car problems.
After that, I found a new hiding spot for the gas. I did however, keep the water filled decoy in the old spot. The best part is that while his car was in the shop the following week, he was getting a ride everyday from another employee. He must have offered to put some gas in his buddie's car, because the can came up missing and the buddie's car died and they both missed a day of work. The shop had a strict attendance policy and because he had so many unexcused absences in a short period of time, he was let go. I don't know if he ever figured it out.
Like Landscaper22 said, just make sure you don't fall victim to your own prank!