It's brown because gas seems to discolor the Tecate tanks from the inside and all the way through. Setting out in the sun for years takes it's toll too. You can sand all the way through most KXT tanks and never get it to original green again.
Try the kreem like Chainsaw said or try to get in touch with a body shop and see if they can die it for you.
1986 Tecate
1984 Tecate
1985 Tri-Z
1986 ATC350X
1985 ATC250R (2)
1985 ATC125M (2)
1982 ATC70 - Original Owner
175 TriMoto Frame with Pro-Tec Rear Suspension
3-Wheelin' since '82