Using a widely used stellar population synthesis model , we study the ability of using pairs of AB system colours to break the well-known stellar age–metallicity degeneracy and give constraints on two luminosity-weighted stellar-population parameters ( age and metallicity ) .
The relative age and metallicity sensitivities of AB system colours that relate to u , B , g , V , r , R , i , I , z , J , H , and K bands are presented , and the abilities of various colour pairs for breaking the age–metallicity degeneracy are quantified .
Our results suggest that a few pairs of colours can be used to constrain the two above stellar-population parameters .
This will be very useful for exploring the stellar populations of distant galaxies .
In detail , colour pairs [ ( r - K ) , ( u - R ) ] and [ ( r - K ) , ( u - r ) ] are shown to be the best pairs for estimating the luminosity-weighted stellar ages and metallicities of galaxies .
They can constrain two stellar-population parameters on average with age uncertainties less than 3.89 Gyr and metallicity uncertainties less than 0.34 dex for typical colour uncertainties .
The typical age uncertainties for young ( Age < 4.6 Gyr ) populations and metal-rich ( Z \geq 0.001 ) populations are small ( about 2.26 Gyr ) while those for old populations ( Age \geq 4.6 Gyr ) and metal-poor ( Z < 0.001 ) populations are much larger ( about 6.88 Gyr ) .
However , the metallicity uncertainties for metal-poor populations ( about 0.0024 ) are much smaller than for other populations ( about 0.015 ) .
Some other colour pairs can possibly be used for constraining the two parameters , too .

As a whole , the estimation of stellar-population parameters is likely to reliable only for early-type galaxies with small colour errors and globular clusters , because such objects contain less dust .
In fact , no galaxy is totally dust-free and early-type galaxies are also likely have some dust , e.g. , E ( B-V ) \sim 0.05 , which can change the stellar ages about 2.5 Gyr and metallicities ( Z ) about 0.015 .
When we compare the photometric estimates with previous spectroscopic estimates , we are shown some differences , especially when comparing the stellar ages determined by two methods .
The differences mainly result from the young populations of galaxies .
Therefore , it is difficult to get the absolute values of stellar ages and metallicities , but the results are useful for getting some relative values .

In addition , our results suggest that colours relating to both UBVRIJHK and ugriz magnitudes are much better than either UBVRIJHK colours or ugriz colours for breaking the well-known degeneracy .
The results also show that the stellar ages and metallicities of galaxies observed by the Sloan Digital Sky Survey ( SDSS ) and the Two-Micron All-Sky Survey ( 2MASS ) can be estimated via photometry data .