You'll find the first one in Aces Weekly Volume 1 No.1. There's page one above. Colin and Semi-Automatic Steve encounter a new menace in the form of the Zombie-Yeds!
The second new complete story appeared in Aces Weekly Vol.8 No.7, with a guest appearance by The Giggly Sisters leading to Colin and Steve encountering their old enemy Madprof!
The third new Combat Colin strip appeared in Aces Weekly Vol.16 No.6, with our heroes flying into space to avert the Ass-teroid!
The fourth new strip appeared in Aces Weekly Vol.21 No.7, and saw Colin and Semi-Automatic Steve battle each other!