The flag of the Greco-Bactrian Kingdom
Like the Macedonian flag (simply the flag of Graece, with only the red background, the sun, and the stripes, which are both completely yellow) and the flag of Makedos, the flag of Greco-Bactria is red with yellow. It is unknown why, but Graece started with it, Greco-Bactria copied it, and Makedos, which wanted to be part of Graece (and became a part of Graece) copied it too. The green, which can be seen on the border, is the color of Greco-Bactria. It is perhaps a bit strange that there is a green border around the flag, for historically, borders on flags represent that the nations of those flags are vassals of others. On the other hand, perhaps this is a reference to the Seleucid Empire, for the Greco-Bactrian Kingdom once was a vassal of it, and Greco-Bactria simply didn't want a purple border (purple is the color of the Seleucid Empire).