  Context. Absorption line indices are widely used to determine the stellar
population parameters such as age and metallicity of galaxies, but it is not
easy to obtain the line indices of some distant galaxies that have colours
available. Aims. This paper investigates the correlations between absorption
line indices and colours. Methods. A few statistical fitting methods are mainly
used, via both the observational data of Sloan Digital Sky Survey and a widely
used theoretical stellar population model. Results. Some correlations between
widely used absorption line indices and ugriz colours are found from both
observational data of early-type galaxies and a theoretical simple stellar
population model. In particular, good correlations between colours and widely
used absorption line indices such as Dn(4000), HgammaA, HgammaF, HdeltaA, Mg1,
Mg2, and Mgb, are shown in this paper. Conclusions. Some important absorption
line indices of early-type galaxies can be estimated from their colours using
correlations between absorption line indices and colours. For example,
age-sensitive absorption line indices can be estimated from (u-r) or (g-r)
colours and metallicity-sensitive ones from (u - z) or (g - z). This is useful
for studying the stellar populations of distant galaxies, especially for
statistical investigations.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics