What I can see in the photos is a revolver in very good (antique) condition. Establishing a market value will require some research, finding out what is currently offered and what has sold over the past year or two. The first step I would recommend is to call Colt (see the website, look for the Colt Historian) and get a factory letter on the old Colt. This will establish when the revolver was shipped and to whom (individual, dealer, police department, etc). For about $150 (major credit cards accepted) Colt will expedite the research, give you a preliminary report on the phone, and have the letter in your hands within a few weeks.

When the gun was made, when it was shipped, where it was shipped, and to whom it was shipped can have a significant influence on desirability and value to collectors. The Colt Historian letter will also identify the original condition when the gun left the factory (caliber, finish, barrel length, stocks, etc), which will also have an effect on value (all original condition is worth more than any gun having been altered or modified). Suggest that you request the Historian address the letter "To Whom It May Concern" rather than issuing it in your name, which will appeal to subsequent collectors in future years.

While that process is going on you can check the on-line auction sites (GunBroker.com and others) as well as major dealers and auction houses, using a simple Google search function such as "Colt Lightning for sale" and "Colt Lightning auction" (you'll get lots of hits, including sales of Colt Lightning rifles and others, so you'll have to wade through some details).

Authenticity and condition will be the key factors in value. You will need to compare each comparable piece carefully to assess how yours stacks up in comparison.

Depending on a lot of variables, I would expect that your Colt will be within a range of value between $2,000 and $5,000 (even that might be low in the event that the Colt Historian letter identifies an original owner with particular historical interest). Getting top value will depend on how you market the gun, what venues you choose to place it in front of as many knowledgeable collectors as possible.