Note when I write "blue item" or any color item it means "item with a blue sales tag".
1) There are 5 color sales tags. Each week there will be a color of the week that at least in my Goodwill goes from Sunday to Saturday where all that colored items are half off.
2) Notice during the week which color items are being put on the shelves. If lets say its all red items most likely that Sunday you will not see Red as the color of the week.
3) You will never, ever see color of the week items being placed on the shelves.
4) Items being taken down, well if most of it is one color good chance that will be the color for the next week. I see that a lot in books. By Sunday there were few blue books on the shelves (books with blue sales tags). Yes this sucks.
5) When the clerks push out carts with stock always dive in before the items are placed on the shelf. If you don't others will. Might as well be first. Savers clerks and Salvation Army clerks always look like they want to rip your spine out if you do this. Goodwill clerks are the coolest about this and will even walk away
Other Goodwill stores may be slightly different but they tend to follow the same pattern.