We present simultaneous $g$, $R_{rm c}$, and $I_{rm c}$ photometry of the notable dwarf nova ER UMa during the 2011 season. Our photometry revealed that the brightness maxima of negative superhumps coincide with the bluest peaks in $g - I_{rm c}$ colour variations. We also found that the amplitudes of negative superhumps are the largest in the $g$ band. These observed properties are significantly different from those observed in early and positive superhumps. Our findings are consistent with a tilted disk model as the light source of negative superhumps.
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