Can you keep a northern pike in Colorado?

No minimum size and no daily limit at all — Colorado actively wants pike pressure kept up, since they are not native and prey heavily on trout in many reservoirs.

CPW source page

What we left out on purpose

• Walleye/saugeye/sauger and white bass/striped bass/wiper both carry a higher combined limit (10 and 20 respectively) specifically on the Arkansas River and the South Fork Republican River. Neither water is covered by this site, so the higher regional numbers are not modelled — the statewide figures apply everywhere this site actually reports on.
• Clear Creek carries a flies-and-lures-only, catch-and-release special regulation from its headwaters near Loveland Pass downstream to Duck Lake — a short alpine stretch far above Golden, where this site's Clear Creek report is anchored. The statewide trout rule is used here because the site's location sits well below that special-regulation stretch.
• This silo covers finfish only. Colorado's crayfish and bullfrog rules (both unlimited statewide) are not modelled, consistent with every other state in this silo.
• Aquatic tiger salamander larvae (a legal live-bait/food species in Colorado under 5 inches, gilled) is not a finfish and is not modelled here.
• Colorado's countless named Gold Medal and special-regulation stretches beyond the four waters this site actually reports on (Deckers, Dream Stream, Eagle River, Clear Creek) are not modelled — the same restraint applied to Louisiana's and Mississippi's dozens of named-water exceptions.