I can see the Blue's 'causes' as something that someone could dedicate their Aes Sedai life to. There are a million 'causes' in the world and in fact all the Aes Sedai's colors fall under 'causes', it's just that their causes are more specific.

I can see Gray's as mediators because there are always disputes on many levels between cities, countries etc etc.

Browns also make sense as people devote their life to being historians, knowledge etc tho why go through all the effort to become an Aes Sedai just to be a historian makes little sense and since they don't usually meet people or deal with people, why bother. The only plus side really is extended lifespan, which means more time to read and or write and special access to the WT's librarys.

White's I can see as being devoted to logic in math, science, astronomy etc, they also don't do much with regular people so also not much reason to become Aes Sedai except extended life & specialized access to other members of the White.

The Red would virtually have nothing to do because there are rarely any men who want to show they use the OP, so unless they spend their lives as Aes Sedai travelling by horse and cart to every city in Randland looking for men who have accidents and don't die, or act funny I can't see why you'd need 1000 of them.

Even more useless is the Green Ajah as they can ONLY fight Shadowspawn, who rarely come to Tar Valon & I never hear about them stationed in the Borderlands which actually is where they should be 100% of the time. If they did their duty Malkier would still exist right now. After all where do they think most of Tarmon Gaidon is going to take place? To me that seems where they should do their battle ajah training as it is the only place where they can actively and freely practice destructive weaves so they can be 'prepared' for TG. However since this is not the case in the books, not sure what they do, and we see that in TGS that the one Green who was fighting the Seanchean didn't do a very good job of it, in fact I believe she was the Ajah's 'Battle General' Hrmph!

Yellows heal, but heal who/where? Unless they are stationed in every city (which they are not) then what good is their 'purpose'?
Can every person travel 2000 miles to bring their sick husband and child to the lazy Healers in Tar Valon? Seems if they really love to heal they should be out and about in the world.

I think all in all most Aes Sedai are lazy and priveledged and don't really perform the actions of Aes Sedai which means 'Servants of All'. Maybe that is why the Chosen laugh at this Age's Aes Sedai since they hardly act like servants, but in fact moreso act like lazy, rich and indulgent rulers.