I've never tried it but I read somewhere that the best way to level up Cloaked, equip Talbot with his Fal and Micro, have your Boosters Cloaked and Monkey Man(?)( I forgot the second booster but just play around with it until you find something you like.)
And for your kickback either use Smoke Bomb or Speedy G.
Smoke Bomb is good for two reasons. You stealth kill someone and by the time you stealth kill them the entire team is already shooting you. So you can Smoke Bomb and chances are you'll get to a good place where you can sneak up on the team again and get more stealth kills and Speedy G is pretty much common sense.
Anyway once you've equipped both of Talbot's guns you should have Stealth Buddy. Get a friend, have both of you equip the guns and go out and level up Cloaked together. I'd do it but stealth kills hate me. I literally only have 18 assassin's and I've been playing U3 since September 2012.
ya Speedy G and Talbot's 3rd booster would help with getting assassin medals or the Gotcha kickback (ability to see all enemy players for 30 seconds, unless they have cloaked too lol)
Only problem with making the colors match the map is you don't know what side you will get or map so it is hit or miss. I got cloaked gold 4 times and after the first 2 it started coming naturally to me. As much as the kal may be good for stealth, I find the micro better, so much that it adapted as my only pistol basically. I personally had an orange Sully and black Flynn, as well as a brown custom hero and grey custom villian when stealthing.
As for Speedy G, yea it is good, but I personally find Gotcha! better for smaller maps since you are already close together, no need to speed up to the enemy.
Maps I find good for Cloaked are Chateau(the dark room with the collapsable roof is amazing), Museum/Yemen(gotta love snipers when stealthing), and ones I avoid are ones with bumpy terrain like Desert Village or highrise, that will make you smack them instead so much.
I actually enjoyed the thrill of leveling boosters so much... I kinda miss it now