My recipe asks for "malt powder" - which do I choose?
"Malt powder" is quite a vague term, as you will read below. The one to choose depends on what the malt powder is being used for in order to discern which the author intends.
A natural improver
Diax (diastatic malt powder) is cream coloured and used in very small quantities (10g per 1000g flour). It is used as a natural improved, giving more lift and a better crumb. Too much and the dough becomes sticky. It is used in malt loaves for example, to increase the sticky texture of the bake.
A natural colouring
Nut-brown malt powder (and others like red malt powder) are not diastatically active and only add colour and a little flavour. These too are used in fairly small quantities (although more than Diax).
A naturally malted sugar
Lastly, there is spray malt, powdered malt extract in 3 shades from cream to dark. These are sugars and are not diastatically active.
Note that for a malt powder to be active, it cannot have been roasted to a high temperature which is needed for the darker colours.