There are three common versions of Boba Fett; Empire Strikes Back (ESB), Return of the Jedi (ROTJ), and Special Edition (SE). Special Edition is referring to Boba Fett's added scenes in the Lucas edits. All look virtually the same to the untrained eye, especially SE and ROTJ. However, each version is almost completely different than the last. This is partially due to having two different painters working on Fett with different styles, but similar visions.
Take a look at the CRLs below...
Most notably, ESB is all green and ROTJ/SE have more color. It goes far beyond that, however.
ESB is all painted by Joe Johnston, and ROTJ painted by Sandy. SE is a mashup of different costumes. When the SE added scenes were filmed the costume designer probably went in the storage and grabbed whatever they thought went together. This led to essentially ROTJ with a ESB helmet, with some minor changes due to damage and staff not knowing what goes where.
The helmet's have different paint jobs, different colors, and different shapes. ESB/SE both use the ESB Hero Helmet. This helmet also has a crooked brow, probably because it was left in the heat and started to warp. The warp is indicated by the blue arrow below. It's subtle, but present.
The paint colors, while still green and red, are very different shades. The back panel is probably the greatest change. ESB is blue with red key slots, ROTJ is green with gray key slots. Also note the extreme differences in weathering.
Overall the entire weathering techniques and styles differ quite a bit. Really the helmets are entirely different, just with similar colors. The ESB helmet is often seen as more favourable, so ESB/SE costumes are more common.
Once more it is similar style, different colors. ROTJ has the same green color with yellow damage across all plates. ESB has a darker color green, with yellow damage on all but the chest plate with the sigal. That plate is a very slight darker green than before with no yellow damage. The dents vary in size and location as well. ROTJ hero has an additional dent on the sigal chest plate, but not the ROTJ stunt. Either is approvable in the 501st. The ROTJ stunt was used for SE so the dent is not there either.
The studs that connect the colar to the back plate have paint on the top of them in ESB, but not ROTJ/SE.
ESB has primarily a deep, burnt yellow for most of its knees and shoulders. The right shoulder bell (with the braids) is a much deeper orange with significant more dents than any other version. ROTJ uses a very difficult to replicate orange color with some burnt orange misting, but with that being said some shots show a much more yellow color that looks more closely to pre-prod 2, but not the same. I prefer the orange color as that is more in line with what I saw in person. The right knee on SE is different than ROTJ because it is a pre-prod 2 knee, so yellow but not the same yellow as used elsewhere. Different color, different dent, different weathering.
The mythosaur skull on the shoulder bell is also a different design. ROTJ is a more refined and cleaner version of the ESB skull.
The dent is virtually the same on the cod, but still has different colors as noted on the torso armor. There is talk of a small dent possibly being on the kidney plate of ROTJ hero or ROTJ stunt, I believe it was decided to just be on hero.