Try before you buy
Ctrl+left mouseclick is your best friend. Using it on a piece of armour or cosmetic opens up the Dressing Room. Here you can preview how the item looks on your character and try out different dye colour options.
Which items can I preview?
You can preview everything that responds to ctrl + left mouseclick. This includes armour and cosmetics on the Auction House, armour icons that appear in the NPC dialogue as quest rewards, items in the sell section on merchants, items in your bags, items from people you are inspecting.
The dressing room is small, can I enlarge it?
If you feel the Dressing Room preview pane is a litte small and want to use a larger preview feature, you can use an interface mod like BDRX.