The System 7.0 UI appearance - before Platinum - was a mess. It was little more than a partially colorized version of the monochrome System 6 user interface; in fact, it mostly fell back to the System 6 appearance on machines with monochrome displays, like the (brand-new in 1991!) PowerBook series.
In a certain sense, Platinum was an attempt to reinterpret what Mac OS could have looked like if it had always been designed for a color display. It didn't just add color, like System 7.0 had; it added depth and texture to the interface which wasn't practical to display before. It also added a ton of new controls to the toolkit which previously didn't have standardized implementations or appearances. (For instance, System 7.0 didn't have a standard progress bar control - every application which used one had to provide their own implementation.)
In a certain sense, Platinum was an attempt to reinterpret what Mac OS could have looked like if it had always been designed for a color display. It didn't just add color, like System 7.0 had; it added depth and texture to the interface which wasn't practical to display before. It also added a ton of new controls to the toolkit which previously didn't have standardized implementations or appearances. (For instance, System 7.0 didn't have a standard progress bar control - every application which used one had to provide their own implementation.)