This ternary (triangular) diagram is the first step in properly classifying the gabbroic rocks. We see that a rock with over 90 percent plagioclase isn't a gabbroic rock but rather is anorthosite. Likewise, a rock with less than 10 percent plagioclase is ultramafic and has its own set of ternary diagrams. In between these limits, the amount of plagioclase pretty much directly determines the overall rock color, ranging between near-white and near-black (with greenish or brownish tinges). Whatever rock name we eventually end up with, this overall color range allows us to add the prefix mela- (for dark) or leuco- (for light) to that name.
If olivine is present among the dark minerals, then its percentage needs to be figured out separately from the pyroxenes and amphiboles (conventionally called hornblende). If olivine is all there is besides plagioclase (95% or more of the dark minerals), then the rock is troctolite. If there's almost no olivine, then we move on to the next ternary diagram, which separates pyroxene from hornblende against plagioclase to distinguish three rock names: gabbro, gabbronorite and norite. If there's an appreciable amount of olivine, that doesn't determine the rock name but requires us to add "olivine" to the rock name that applies.