To change the color of the player on the left (right), you press F1 or F2 (F3 or F4). To select a computer opponent, you press F5 or F6 which actually selects the computer level (1 is the easiest, 9 is the hardest). For people that don't like color, you can press F7 or F8 to switch to monochrome players (great in the days of monochrome monitors).
International Soccer remained the best footie game on the Commodore 64 until Matchday 2 and Emlyn Hughes International Soccer came along.
This is me playing in white against the computer in black at level 5.
The problem with this game is that you don't have enough control of where the ball is going (and how), for example, you can't pass the ball on the ground, which is real annoying. Also, the heading AI is a bit over the top: one can pretty much cover half the field's length heading the ball. All in all, a very good game at the time it first came out.