As marked, its a Colt New Service revolver, which was Colt's large frame revolver.
The New Service was made from 1898 to 1942.
It was offered in various barrel lengths from 4" to 7 1/2", and in finishes of bright blue and bright nickel.
It was also made for military use as the Model 1909 in .45 Colt and Model 1917 revolvers in .45ACP.
The commercial models were made in just about every pistol cartridge made during that time.
You can determine the production year by checking the serial numbers here:
http://proofhouse.com/colt/
This one may have been re-blued since it looks "too good". The grips are the original hard, black "Gutta Percha" rubber as used up until the early 1930's with checkered walnut after that.
Value depends entirely on actual condition, and whether its been refinished.
As a ball park figure, assuming its in 95% original condition, somewhere around $1200 or more.