When you type /shirt start, the addon switches through all the different coloured shirts in your inventory. It will stop when you type /shirt stop.

For this to work you must have the following shirts in your inventory:

Common White Shirt, Brown Linen Shirt, Bright Yellow Shirt, Stylish Red Shirt, Black Swashbuckler's Shirt, Gray Woolen Shirt, Green Linen Shirt, Stylish Blue Shirt, Pink Mageweave Shirt, Orange Mageweave Shirt, Lavender Mageweave Shirt.

When you have all the shirts and you've started the switcher, it will appear that your shirt is flashing many colours really fast.