\, .\label{eq:main}\end{aligned}$$ The main correlation also explains the FIR colours of Galactic high latitudes ($|b|>5^\circ$), the LMC, and the SMC (@hibi06 [@hibiphd06]; HHS07). In the Galactic plane, there is another correlation sequence, called subcorrelation in @hibi06: $$\begin{aligned}
(140/100)_\mathrm{cl}=0.93(60/100)_\mathrm{cl}^{-0.56}
\, .\label{eq:sub}\end{aligned}$$ The subcorrelation is not seen in the high Galactic latitudes (@hibiphd06; HHS07). This supports the idea of @hibi06 that the subcorrelation is produced by a contamination of high-ISRF regions, which tend to reside in the Galactic plane.

![Far-infrared colour–colour diagram showing the relation between 60 $\mu$m–100 $\mu$m colour and 140 $\mu$m–100 $\mu$m colour. The filled squares with error bars indicate the current BCD sample, while the crosses and open diamonds show the DIRBE data of the LMC and the SMC, respectively, taken from @hibi06. We also present the Milky Way data (with Galactic latitudes of $|b|<5^\circ$) adopted from @hibi06 by the contours, which show the levels where 50, 80, 90, 95, and 99 per cent of the data are contained. The dashed line and the dotted lines present the fittings for the main correlation (equation \[eq:main\]) and the subcorrelation (equation \[eq:sub\]), respectively. \[fig:clrdat\]](bcd_clr.eps){width="8.5cm"}

It is interesting that not only the LMC and the SMC but also the current BCD sample 