  The observed colour bimodality allows a classification of the galaxies into
two distinct classes: the `blue cloud' and the `red sequence'. Such
classification is often carried out using empirical cuts in colour and other
galaxy properties that lack solid mathematical justifications. We propose a
method for separating the galaxies in the `blue cloud' and the `red sequence'
using Otsu's thresholding technique for image segmentation. We show that this
technique is insensitive to the choice of binning. It provides a robust and
parameter-free method for the classification of the red and blue galaxies based
on the minimization of the intra-class variance and maximization of the
inter-class variance. We also apply an iterative triclass thresholding
technique based on Otsu's method to improve the classification. The galaxy
colour is known to depend on the stellar mass and the luminosity of galaxies.
We obtain the dividing lines between the two populations in the colour-stellar
mass plane and the colour-absolute magnitude plane by employing these methods
in a number of independent stellar mass bins and absolute magnitude bins.
