Matcha powder
Finely ground Japanese green tea, bright green, bitter and grassy with a slight sweetness.
Matcha is sold in Asian grocers, health food shops, many drugstores and online; for cooking and baking the cheaper culinary grade works well, and fresh, vivid green powder is better than dull, olive-coloured powder. There is no equivalent substitute, because finely ground loose-leaf green tea stays gritty and barely colours, while colouring powders give the colour but not the flavour.
Substitute:
No good substitute: finely ground loose-leaf green tea gives some bitterness but stays gritty and barely colours.