There are a couple of things that will cause your gas to look red or orange in color. One of them is someone has put fuel stabilizer in it. One particular fuel stabilizer that goes by the brand name “Stabil” is red in color while their Marine Grade is a green color. Too much Stabil added to your gasoline will cause it to look red.
If your lawn mower runs fine, then the Stabil is doing its job and keeping your gasoline stabilized. A fuel stabilizer prevents the gas from separating into different components and drying up in your carburetor bowl. This process of phase separation degrades the fuel and it begins to go bad in as little as 90 days.
If however, your lawn mower doesn’t run at all, there is a good possibility that you have off-road diesel in the tank. Off-road diesel is red in color on purpose. This is only meant for things like construction equipment and equipment that is not driven on the road, hence the name off-road.
You will need to drain all the diesel out of the lawn mower and put fresh gasoline in and the lawn mower should fun fine once again.