I use coloured plastic straws cut into shortish lengths. Yes - really!
For example, in the GH when growing more than one variety/colour of the same plant, one label +pink straw = Tomato, Gardener's Delight. All pots with pink straws are therefore effectively labelled at very little cost. One label +blue straw = Tomato, Marmande. All pots with blue straws are quickly identified and don't get confused with other tomato plants.
Works with all sorts of plants (white and pink lavatera, for example) where the seedlings are easily confused.