I recently went on a fishing trip to the South Platte river between Spinney mountain reservoir and Eleven Mile reservoir(gold medal water) in Colorado. My dad and I caught a couple of small brown trout. The fish had extremely vibrant colors and were very pretty. I believe that they were wild, but I am not sure. So I assume that wild trout are harder to trick into striking, but is there a difference in fighting, average size, coloration, and so on between wild and stocked trout?

Once stockers have been in the water for a while they lose their baby fat and rounded fins from scrubbing concrete and become hard. After they've been in there for a year or so, you'd be hard pressed to tell the difference aside from the fact that stockers tend to eat a little more readily, IMO.

Only way I can tell is the fins and taste.  As stated, the stockers fins are normally ground down and it takes a while for them to fill all the way back out.  Also they taste "grainy" when eating them.  That said, this is for Rainbows, as we release all browns regardless of size around here.

 

Jeff

You have to look at their pectoral fins, if they are rounded off it's a stocked fish, sometimes it's hard to tell because the fins heal over time after stocking but they never will look as good as the pectoral fins on wild trout. 

One differences is that the stocked trout taste like dog food

Many times they will actually cut a fin off a stocked fish so you can tell the difference.  I have found that wild fish eat more readily than stockers but are more skittish.  In general they live a short life where food isn't always available so they take what is presented to them....now this is in smaller streams that i have fished on both the east and west coasts but not in CO specifically.

One differences is that the stocked trout taste like dog food

 

that is just the freshly stocked trout, once they've seasoned in river eating natural forage they taste pretty much the same.

 

one of the easiest ways to tell the difference especially on that river is the Adipose fin between the dorsal and tail is clipped before it's stocked.  that stretch of river you fished is Gold medal and you are fined heavily for poaching.  that leads to a larger quantity of wild.  I've caught both from that stream.  Usually the color on the stockers is duller, and they are more beat up, partly due to the conditions in the hatchery tanks and competing for feed.  The wild trout are more territorial and don't usually invade another trouts space.

 

The wild fight harder, but it depends on where they are from, the time of year(trout are much more sluggish in the hotter months), and size.  That section is hit hard by anglers so the wild are no easier to fool than the stocked.  if they've been there longer than one season they have seen every fly known to man, and every presentation known to man.  I got there one morning on the South Platte and was the first angler there, by 9 AM i looked behind me and counted 20 anglers within a few hundred yards. The wild in many small mountain streams are usually easier to catch because they see fewer flies and have to take every opportunity to feed.

 

here are few pics i took out there last year.