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Hello Syafiq,
As you know, caffeine belongs to xanthines which in turn belong to alkaloids. When the Dragendorff reagent is used, the presence of an alkaloid is indicated by formation of red-brown precipitate. As you know also, pure caffeine is white while crude caffeine is nearly pale yellow so if it precipitates then it will come in this/these color(s) with the color(s) that come(s) from the inorganic product(s).
Anyhow, the caffeine really has an "unusual" behavior: it does not dissolve in cold water & dissolves in boiling water (that is what is done when we make coffee or tea). When the beverage cools down, caffeine does not precipitate. This compound's response to the (D test ) may be another "unusual" behavior.
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