I am not purple, but I was instructor in BEPRO - War Acedemy and have some experience with mentoring. Now our students have own clan (its possible to join only after finishing BEPRO - every single member was our student). Most of them started with orange/yellow recent and even worse overall: http://wotlabs.net/eu/clan/IMPR0
First of all you have to face the most important - it is IMPOSSIBLE to teach anybody to play this game. The only you can do is to show how to learn to play this game. If somebody do not want your help - dont waste your time. Its pointess. Use it for somebody, who wants to use it.
So - i believe that every single player can reach green recent (out of really special players) only if want it and have decent mentor.
Imho the best method is playing platoons with player with at least 1k recent more and is explaining whats happening. Which tier/tanks? Depends of students actual level. I would start with t-34-85 and Cromwell (or premium versions of them).
Focus on this elements:
-reactions for what happens on minimap - this is the most important
-understanding role of the tank in battle - depending of tank class, map, MM (top or bottom)
-mechanics! - penetration mechanics, spotting (understanding 15m, proxy spot, spotting 445m harcap, drawing)
-prepairing tank for battle - modules, equipment, crew (what perks and how many perks on which tier they should have)
Point the biggest mistake - explain why it is mistake and how to improve. Use just finished battle as an example. After session - let the student work with it. Come back after few days and check if problem still exist. Help to eliminate it or focus on smaller mistakes.
Some of players will need time to improve. Some of them will need only few battles. I was mentor of my favorite platoon mate. He just needed few tips and just went just like that from 1800 recent to 3000.