Aside from Cataclysm 2nd, I've recently returned to the world of mobile suits and such...
I present a re-design of the Leo and its earlier incarnation... making the differences a little more distinct than panels in the armor.
Left to right, top to bottom:
While the Earth Federal forces struggled with Project V to match the Zeon's new Mobile suits, the Romafeller foundation managed to devise their own mobile suit design from stolen Zaku plans. The result was the OZ-05MS "Leo" design. This is the coloration of the first sucessful mobile suit unit from the Romafeller foundation.
As the Leo was made production-ready several weeks in advance of the EF's GM lines, a number of these pre-production models ended up in the hands of Federal pilots, many of whom would switch to the GM when it became available. Some retained their leos, however, citing the Leo's ability for better mobility and ease of repair.
These are the rollout colors for the "Early Leo". Two batches of a dozen units were produced before production switched to the more practical "Leo 2nd" design.
The "true" Leo, this was the design the Romafeller foundation was seeking to perfect. It began production around the same time as the GM, and there was competition between the Earth Federal in-house suit production and the suits produced by the Romafeller foundation. While GMs were generally better armored, Leos were easier to mass produce as they used off-the-shelf components, rather than the Gundam-derived parts of the GM line. The Leo, like the GM, would remain one of the popular mainstay suits for years to come.
Still using the Romafeller foundation's model code, Karbara and the AEUG managed to acquire a number of Leo Mobile suits for their war against the tyrannical Titan organization following the One Year War. They were quite popular in the PLANTs of Side three, as the GM was seen as a symbol of the Blue Earth movement.
The Titans managed to aquire a few stock Leos for use among their forces, but not many of them saw action. As the Titans were interested in mass-producing a Gundam-derived line, the Leo wasn't as popular among the pilots as the GMs were.
OZ-06MS-C Kushrenada custom
During the middle months of the war, Oz representative Treize Kushrinada was given an OZ-06MS to use whenever he lead a battle personally (which was frequent). His unit typically stood out among other Leo units, and was occasionally confused for a GM at a distance. Treize would continue this color scheme when he eventually upgraded to the second Tallgeese prototype.
OZ-06MS-C Zechs custom
Ace pilot Zechs Marquise was also given one of the first C-model Leos to use personally in battle. Zechs used it only once, shortly after it was built, before facing Zeon's new mobile suits and discovering the prototype OZ-00Ms Tallgeese.
OZ-06MS Quattro Bajeena custom
Under the alias Quattro Bajeena, newtype Zeon ace Char Aznable infiltrated the OZ organization and, after demonstrating his combat ability in an OZ-05MS against a trio of mobile dolls, was given his own Leo. He would use the Leo (and his alias) to follow the White-Base class Archangel across the earth before abandoning the name and the suit to return to space.