I think it was a 1970s novel and might have been simply titled "Colony". These days it would likely be classed as young-adult, but I don't think such a category existed back then. I probably read it in the 80s and I remember the paperback having a mostly purple cover.
The overall story was similar to Anne McCaffrey's Sassinak. It started out on a colony planet in the far future with two main characters - a teenage boy and girl. Space pirates attack the planet and I think take the boy hostage. My recollection is that the majority of the plot was about him eventually fighting his way back, but there were a couple of subplots also.