usually the yellow tag is the final clearance price. they stack the red tags as it gets marked down more and more then put the "get-it-outta-here" yellow price. I've found that the markdown cycle is usually correlated w/ the number in the square box on the price tag that indicates when they got the item. the longer it's there, the more it gets marked down. sometimes not everything gets discounted consistently however. for instance if all the ties marked down are numbered 22, and you find one you like that's a 22 but not marked down, it was probably missed and if you ask, they will usually discount it for you. a couple times a year all the REALLY leftover shit gets marked down to like $3.
Also their "higher end" stuff gets a purple price tag. Generally this means that these items will be passed over for reductions as they are feeding off of brand name, etc. I find that a lot of the purple tagged stuff, especially the more expensive stuff, will sit there for a week or two after initial mark downs before it is given its due.