At my DC school they are asked to wear a coloured t shirt for sports day. They are given a colour eg blue, green, yellow, red. We are given lots of notice for this. Sod's law, the colour my DD has been given is the one colour she doesn't have already.
I know from parents of older children that each year the children are given another slip asking for another t shirt, OK so kids grow but wouldn't it make sense to keep the same colour throughout their time at school? Most parents I've spoken to just buy a fruit of the loom top from Amazon at £1.50 but it seems quite wasteful. It's a big school so potentially over 100 t shirts could be being bought for sports day each year. I've found a bundle of second hand t shirts on ebay including the colour we need, in the next size up so they can all be worn again.
Disclaimer: I watched Years and Years yesterday so Gran's rant about £1 t shirts is fresh in my mind!