\begin{table}%t6
\caption{Average colours of the host galaxies$^{{a}}$.
\label{col_average}}
%\centerline
 {\begin{tabular}{lccccc}
\hline\hline
\noalign{\smallskip}
The sample & $U-B$ & $B-V$ & $B-R$ & $R-H$ & $B-H$ \\
& & & & &\\
(1) & (2) & (3) & (4) & (5) & (6)\\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
BL Lacs (this work)        & $0.3\pm0.4$ & $0.8\pm0.2$ & $1.2\pm0.5$ & $2.2\pm0.4$ & $3.5\pm0.5$ \\
Quasars \citep{jahn04}     &             & $0.5\pm0.1$ & $1.1\pm0.1$ & $1.9\pm0.2$ & $2.9\pm0.3$ \\
RGs \citep{govo00}         &             &             & $1.4\pm0.2$ &             &             \\
Ellipticals \citep{pele90} & $0.6\pm0.1$ &             & $1.6\pm0.1$ &             &             \\
Ellipticals \citep{bowe92a}&             &             &             &             & $3.8\pm0.2$ \\
Ellipticals \citep{colb01} &             &              & $1.5\pm0.2$ &             &             \\
Ellipticals \citep{fuku95} &             &  1.0        & 1.6         &             &             \\
Ellipticals \citep{fioc99} &             &             &             & 2.2         & 3.8         \\
Spirals (Sb) \citep{fuku95,fioc99} &       &  0.7        & 1.2         & 2.1         & 3.3            \\
\hline
\end{tabular}}
\par
\smallskip
$^{a}$
Column (1) gives the sample; and Cols.~(2)--(6) the average $U-B$, $B-V$, $B-R$, $R-H$ 
and $B-H$ colour, respectively. 
\end{table}