These hand size/shaped sunfish typify the family.  Mention sunfish and there’s little doubt that bluegill with come to mind.  Their bluish-black earflap is a distinguishing feature.  Equipped with a relatively small mouth, they are nonetheless voracious feeders taking surprisingly large lures. The spiny dorsal fin usually has ten spines and the anal fin three. Bluegills are usually dark olive green along the back, with a variety of colors along the sides, lavender, brown, copper, or orange.  Older specimens often have a reddish-orange or yellow belly.
 
Spawning begins when water temperatures near the70’s. Typically starting in May or June and they may spawn all summer.  As a result bluegills often overpopulate. They prefer to nests in shallow water on a gravel base. Males, like most sunfish, guard the nest. 

Ahh the joys of childhood and catching bluegills on a bobber and worm...at least that's how it was for us. But this hard-fighting panfish has a dedicated following, more so than just child's play. So chances are there's a body of water very close to you that is stocked (and stacked) with bluegill. The world record angling record tips the scales at 4 pounds 12 ounces, and adults can grow over 12" long. Get one of these hooked onto your lightweight fly or spin rod and it'll definitely take you for a spin! And while you're at it, take a kid out who could use a little fishing time!

From the CPW:
This sunfish has a short and deep body. As with all sunfish, the dorsal (top) fin is not split. The bluegill has a small mouth on a short head and a dark gill flap with no trim. There are parallel vertical bars on the side with long, pointed pectoral (side) fins. A male bluegill in breeding colors has brilliant blue fins and a red-orange stomach. The female bluegill is dark on the back with vertical stripes on the body. Bluegills are best caught in the morning or evening using small tackle ranging from a bobber and worm to delicate dry flies. Once one bluegill is located, others will be nearby. Bluegill spawn in colonies from late spring to August, building nests on gravel, sand, mud, leaves, or sticks in 1-4 feet of water. As summer heat becomes extreme, these fish move to deeper water and the shade of weed beds. On Green Sunfish: This fish is similar in appearance to the bluegill, but has a larger mouth and is olive in color with short, rounded pectoral fins and yellow trim on the fins. This stocky fish is found in both streams and impoundments and spawns in shallow areas from June to mid-August. Like most sunfish, this sporty panfish can be taken with crickets, worms, and other bait rigged under a bobber, or with small lures, jigs, and flies.

Bluegill in Colorado can grow large and are plentiful in numerous ponds and lakes throughout the state.  

State records as of 9/2023:
By weight: 2.59lbs at Totten Lake in 2019 
By Length: 13.5 inches at Sawhill Ponds (prior to 2020 Master Angler program revamp)

Colorado Master Angler Award qualifying length for bluegill is 10 inches.