On June 8, 1977, just a week after the TV Game 6, Nintendo released the Color TV-Game 15 (カラーテレビゲーム15 Karā Terebi-Gēmu Jū Go). With the two controllers now on cables (making for much more comfortable play) and 15 slightly different versions of Light Tennis, the CTG 15 yielded sales of more than one million units. Two models of the CTG 15 were released, differing only by a slight color tint change. The Color TV Game 15 included two variations of Hockey, Volleyball and Tennis, and two ping pong style games. Each game could be played in singles or doubles mode. The extra game is Penalty Shootout where the object is to shoot the ball past a constantly moving target. Also, the paddle controls are extended apart from the console, and wired into it. The console was produced in two shades of orange; the light orange one is much more uncommon. Nintendo sold more than one million Color TV-Game 15 consoles.
It appears as an "Assist Trophy" in Super Smash Bros. for Nintendo 3DS and Wii U, and Super Smash Bros. Ultimate.
Source: Wikipedia, "Color TV-Game series", available under the CC-BY-SA License.