The standard manifold air temperature gauge has the following four colour bands:
Red. This smeans the air is too hot and there is a danger of detonation. Carb heat must be turned off.
Green. This means that air temperature is within safe limits. The air temperature is such that neither icing nor detonation are likely. If the carb heat is on you must leave it on or you might go into the yellow band. If it is off you must leave it off or you might go into the red band.
Yellow. This means that icing is probable. The carb heat must be turned on to get back into the green band. If you are in the yellow band and heat is already on it might be beneficial to turn it off to go into the white (uncoloured) band.
White or uncoloured. This means that the air is too cold for carb icing. If the heat is off leave it off or you might go into the yellow band. If it is already on then the OAT is pretty damned low and you should probably be reassessing your reasons for flying today!!!
In a float type carb system the air temperature in the inlet manifold is reduced for two reasons:
Firstly the choke tube is a venturi. It is intended to reduce static pressure to draw in the fuel, but it also reduces temperature (static pressure and temperature tend to go hand in hand).
Secondly in order to evaporate, the fuel draws heat from the surrounding air (latent heat of evaporation. This cools the air.
Both of these factors cause the air temperature to reduce and if it gets low enough and there is sufficinet moisture in it, then ice will form.
In fuel injected systems the fuel is injected into the inlet ports within the (very hot) cylinder head. This elimates the problem of cooling due to evaporation but not cooling due to acceleration of the air in the manifold.
The magnitude of the pressure and temperature drop caused by the choke tube is proportional to the degree to which the air accelerates. At low power settings the gaps around the throttle valve are very small so the acceleration around it is greater. This high acceleration causes a big drop in static pressure and temperatre.
The danger of icing is therefore greatest at low power settings.
This whole subject was publicised by the CAA some time ago in AIC 133/92 (Pink 68). Most of the JAR questions and FTO notes on the subject are based on this document.