How to wire switched light fixture in place of ceiling fan
Typically there are two hot wires coming into a fan box that has separate switches for the fan and light kit. Typically the black is used for the fan, and the red is for the light kit. That said, other color combinations are possible. 

Typically, the blue wire from your fan/light combo supplies the light kit. Whichever of the two hot wires was connected to the blue fan/light wire is your huckleberry. 

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