Green Sunfish
The green sunfish has a large mouth and a heavy, squarish thick body shape. Their gill cover is dark bluish-black typically with a yellowish-orange margin.  Body coloration is dark green to bluish-green with a yellowish underside.  Fins frequently yellow edged.
 
Greenies are a versatile species, tolerating a wide range of environmental conditions.  They nest in shallow water in tightly packed colonies. Gravel bottom sites are preferred. When water temperatures rise above 70°F spawning begins, and may occur throughout the summer. Green sunfish frequently hybridize with other sunfish, such as bluegills. After the eggs are deposited, male defend the nest for approximately a week. 
 
A prolific species, these sunfish frequently overpopulate in small lakes and ponds, resulting in stunted fish. As a result green sunfish rarely reach a desirable size for angling, with a typical fish being 3-6 inches.  The world record is 2 pounds 2 ounces.
 
Green sunfish are highly predaceous taking just about anything small enough to fit in their mouths.  Insects, small fish, small crayfish, and frogs are all on the menu.  This makes them one of the easiest fish to catch and an excellent fish to teach kids the basics of fishing.

Courtesy of NDIS Natural Diversity Information Source, Colorado Parks and Wildlife

More tolerant of silty water than other sunfish, the green is one of only two members of the family native to Colorado. This stocky fish, found in both streams and impoundments, spawns in shallow areas from June to mid-August with the male guarding the nests. The green is a sporty panfish taking crickets, worms and flies. Most green sunfish tend to be small, ranging up to 8 inches with a tendency towards overpopulating and hybridization with other sunfish species.

Status: This species is not listed.

Colorado state records

Kept           11.6 inches, 1 lb 5 oz
Released  11.5 inches

Master angler qualify in length is 10 inches