Platinum is also the name of Apple's "Classic" appearance (window shading, grey Finder bar, the Apple Menu, etc). The traditional appearance appeared in all OS's (Systems) before OS X. The new appearance is called Aqua.

Platinum is thusly named due to it's dull grey color, but in all OS's above 8.5 (or 8.1 if you have Kalidescope), you can "skin" them by applying custom themes. This is an appearance manager function. You get this for free if you use the appearance manager API when writing an app. Mac OS has looked the same for ages. It is a huge risk for apple to change it now.