Answer
At full power the light looks white, at lower power red light is emitted.
Work Step by Step
At full power, the filament is hot enough to emit EM radiation across the entire visible spectrum, so the light looks white. At lower power, the filament temperature is lower. As a result, the peak of the emitted EM radiation is shifted toward lower frequencies, which increases the relative amount of red light in the mixture relative to other colors.
