Hi Tony Thanks for asking about tangs before you purchase.
IME Emily gives good advice.
Avoid any ancanthurus species like the powder blue, powder brown and the sailfin and atlantic blue tang as they require the larger longer tanks that your 90 does not provide for.
Further the powder blue tang although very beautiful is an extremely difficult tang to keep even by experts. Most often they up and get diseased for no apparent reason and die often taking their tankmates with them.
I would say the kole is the best for your tank or the yellow. The purple is a beautiful fish but very aggressive. If it is your choice only put her in as the absolutlely last fish.
Take a look at
www.wetwebmedia.com/tangs,.htm which will give you excellent information on keeping tangs and in particular which ones to avoid.
Remember never never buy a tank online as you cannot see what fish you are buying. You need to see the fish eating. Do not believe the LFS if they say it just ate. It must be plump, free of spots or tears and alert with clear eyes. Anything less than that, pass on the fish as there will always be another.
Avoid buying any tang which is caught in the Phillipines or Indonesia as the chances are high that it was cyanide caught. Ask to see the country of origin of the fish you buy. Soon we will all be able to buy certified net caught tangs so you might want to wait a while.
Lastly do a seach under garlic extract and follow the feeding regimen. Garlic, IME, acts as an ich preventative and should be used when adding any tangs as they are often ich magnets.
Good Luck