Google calendar allow to affect colors only to calendars, NOT TO EVENTS.
So, you have to split your events on multiple calendars to have different events colors.
To change calendar colors :
New Google calendar individual event color change is not yet handled by Android and by my app.
For Exchange calendars, you can use Calendar Custom Color tool to change calendar colors.
Note : Changing calendars colors can have strange effects with some Android version. After each change, the color on Android google calendar (and in the widget because they share the same color) can become darker and darker. A solution is to clear the ‘calendar storage’ datas in android settings, it will force a full resync of android calendar datas.