Distribution of the colour indices (Y − J) and (Z − Y). The upper panel show all detected PNe without applying any extinction correction. Other symbols show other emission-line stars, main sequence stars (Hewett et al. 2006) and white dwarfs (taken from the standard database of MKO) as indicated. The bottom panel shows dereddened colours for the PNe, using the reddening measured from the hydrogen lines. The distribution of the colour indices (Y − J) and (Z − Y) for a model grid of planetary nebulae is shown in blue.