Unless you are planning on grinding carbide, you do not need a green wheel. An aluminum oxide (especially the white, a more pure form of AO) wheels will do just fine.
Green wheels are more commonly used on "carbide grinders" where you grind on the face of the wheel, not on the edge. Your grinder is a more common variety.
7" wheels are heavily used for surface grinding. Be careful and look for the correct arbor size, a standard 1.25" arbor hole for a surface grinding wheel won't fit your grinder without a bushing. Your grinder probably takes a 5/8" arbor size wheel, but that's just a guess.