Individual differences in cognitive performance in childhood are a key predictor of significant life outcomes such as educational attainment and physical and mental health. Differences in cognitive ability are governed at least in part by variations in brain structure. However, studies commonly focus on either grey or white matter metrics, leaving open the key question as to whether grey or white matter microstructure play distinct roles supporting cognitive performance or if they are two ways to look at the same system complementary. To compare the role of grey and white matter in supporting cognitive performance, we used regularized structural equation models to predict cognitive performance with grey and white matter measures. Specifically, we compared how morphometric grey matter measures (volume, cortical thickness and surface area) and indices of white matter microstructure (volume, fractional anisotropy and mean diffusivity) predicted individual differences in general cognitive performance. The models were tested in a large cohort of children (ABCD Study, N=11876) at 10 years old. We found that grey and white matter metrics bring partly different information to predict cognitive performance. Indeed, model selection approaches consistently demonstrated both tissues were needed, compared to simpler models with only grey or only white that explained respectively 12.3% and 10.9% of the variance in cognitive performance, the combined models explained 15.4% of the variance. Zooming in we additionally found different metrics within grey and white matter had different predictive power, and different regions for grey and white matter had the strongest association with cognitive performance differences. These results show that studies focusing on a single metric in either grey or white matter to study the link between brain structure and cognitive performance are missing a key part of the equation.