Oh man, where to start...
One year of lettuce means it was poorly fed, meanng that there is a good chance it has MBD (metabolic bone desease). Check the jaws, if they are swollen and "spongy", as well as for "Popeye" arms and legs. They seem to be swollen, as if on steroids, where in fact they are deformed. Also look at the back and tail for deformities or irregularities.
It has to learn to feed on good food, not junk food. However, it's an animal, so it does not happen overnight. Mince the lettuce with the appropriate vegetables it needs for a proper diet. In the beginning, so 90% lettuce, 10% good veggies. Slowly progress to include more of the healthy food, lessening the amount of lettuce. By the time you get to 50/50, keep it for a while longer, so that the animal gets used to the taste of the healthy food. This will all happen according to the individual animal, but stick to your guns, and trust me: if it's hungry, it will eat anything. Beans (cooked), particularly red beans, are great as an addition to the mix. You can press them to make them soft and easily mixed with the food. Carrots, finely minced, are also another great addition.
How big is the iguana, and how big is your current enclosure? DO give it some time out of its enclosure, for free roaming, and good muscle workout. If it is still small, make sure the room is "iguana proof", since they can wedge and sneak into anything when small.
Do plan to give it a better, bigger enclosure. The bigger it is, the better for the animal. Make sure to have all the proper lighting. You did not mention a UV lamp, which you need to get (also called Vitalites, or UB bulbs, etc). These will simulate sunlight, and help synthesise Vitamin D3 naturally to absorb calcium. Do not have these too close to the animal, since it can cause retinal and skin burns, and do not have them on 24/7. Give the animal a normal day/night period. Ceramic heat lamps or red-lights can be used to keep the animal warm, but not wide awake.
If the iguana does have MBD, it can be treated and reversed. Give it also vitamin and calcium (that has included D3 for absorption) supplements. Do not give it everyday, once or twice a week depending how grave the animal's condition is.
Reason for its weakness is probably weak muscles and weak bones. It can all be fixed, but with time, patience, and lots of care.
Since it was cared for poorly, chances are it never saw a vet, and therefore, chances are it has endoparasites to take care of. Therefore, you really really have to have it looked at by a vet (endoparasites, as in worms, etc.) Lethargy and discoloration are due to lack of proper lighting, diet, activity, space, and internal physical health.
Try to immediately include more nutritious foods + supplements in the diet as mentioned above. Try to "trick" it by adding different flavors.
We all got our "feet wet" with Melissa Kaplan's venerable wesbite, http://www.anapsid.org/iguana/icfs/index.html
This is a great, detailed crash course into everything you need to know about iguana care. Her book, Iguanas For Dummies, is somewhat a repetition of the website, with some new material and info. It is not too expensive, and highly recommended. My favorite book is by James Hatfield, called The Ultimate Iguana Manual, but it will cost you double, and has pretty much the same info as the other. Avoid the $5 book sold in petstores by Richard Barlett. It is outdated, and shamefully still sold at a bargain price to novices. It does have a few good pointers, so for $5 if you want to have at the moment some references at hand, you can buy it, but ignore the diet chapter. However, you could wasily print all the pages from the website I provided above (roughly 60 I think) and have yourself a perfectly fine "manual".
Skim through this Iguana forum for additional information. Go through past pages, or even do searches on key words that might interest you. Chances are if you got other questions, they already got answered and are there for you to look at. However, keep in mind there are no such things are stupid question, so no matter what question you have, feel free to ask without worries.
You got yourself quite a basketcase of an iguana!
But don't worry, I've seen iguanas in near-death conditions that made it once they were properly cared for.
Your priorities are a good visit to a Vet, and getting it to eat right. Be persistant, always have it freshly made, and ALWAYS keep the enclosure clean DAILY! The weaker the animal, and the dirtier the enclosure, the more risks of desease for it, and salmonella for you! Also, get your UV light(s).
After all that, start planning on a better, bigger enclosure.
The website I gave you does mention iguana-proofing your room(s) and your home (the books do too). Free-roaming is one of the best things you can offer to an iguana. It stimulates it mentally, and physically.
If you live in a warm climate, like Florida etc, where it is warm nearly year-round, plan for an outdoor enclosure. That is the ultimate "dream home" for an iguana, and the best way for full recovery. Natural sunshine, fresh air, large-size potential, and endless possibilities of adorning and building it.
Good luck with it, and congrats on taking the hefty task of caring for a sick exotic animal!
Let us know your progress, etc. and if you have other questions, keep asking!
-IJ
