on methods using red IRAC colours, such as by @lacy04 and @stern05, should be used with caution and a clear understanding of this important caveat.

Systematic colour difference between SMGs and AGNs
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Both SMGs and AGNs appear within the broadly defined red IRAC colour regions by Lacy et al. and Stern et al., but SMGs and AGNs are distinguished by clear systematic difference in their mean IRAC colours. The SMGs as a group are systematically bluer than the AGNs, with only about 20% of SMGs appearing mixed in among the power-law AGNs in both diagnostic colour–colour plots. All AGNs show log($S_{8.0}/S_{4.5}$) $\ge0.2$ in Figure \[fig:colourcolourplot\] while the overwhelming majority ($\ga80\%$) of the SMGs have a smaller flux ratio. Similarly, all AGNs have \[5.8\]–\[8.0\] colours redder than +0.2 in Figure \[fig:colourcolourplot2\]. The most significant colour difference is between the 4.5  and 8.0  bands with an average difference of $\langle[4.5]-[8.0]\rangle\ga 1$ mag. The \[3.6\]–\[4.5\] colour difference is the smallest, with $\langle[3.6]-[4.5]\rangle \sim 0.25$ mag.

The comparison of the full IRAC band SEDs shown in Figure \[fig:IRACSEDs\] offers a visually compelling demonstration of a steeper spectral slope for the AGNs. The average IRAC SED of the AGN sample is in excellent agreement with the empirical type-2 QSO SED template by @polletta07. The template SED for a type-1 QSO is nearly identical (not shown) to type-2 QSO at these wavelengths, and the steep spectral slope is characteristic of all obscured and unobscured AGNs. In comparison, the average IRAC SED of the SMG sample is significantly flatter, with only a few outliers with an AGN-like steeper spectral slope. The ULIRG Arp 220 SED redshift to $z=2$ offers a far better match to the SMGs, lending further support for the dust obscured starburst interpretation. Furthermore, the flatter spectral slope (bluer IRAC colours) suggests that SMGs in general do not host a dust-obscured, energet