You would want to have at least a 12g extension cord, preferably a 10g. Color of extension cords do not matter, that is just the insulation color of the given cord. You should also be fine with an extension cord that is 50' or less, see below:
How far can you run wire before voltage drop?
The following are the maximum lengths of cable you can use while still maintaining a 3 percent voltage drop for the given wire size (AWG) and circuit voltage. As an example, for a 120-volt circuit, you can run up to 50 feet of 14 AWG cable without exceeding 3 percent voltage drop.
Source: www.thespruce.com