You can add custom CSS class to events in BeforeEventRender, depending on the conditions, e.g. "complete30", "complete70", "complete100".
Then define the CSS rules (an example for scheduler_green theme):
.scheduler_green_event_bar_inner
{
position: absolute;
height: 2px;
}
.complete30 .scheduler_green_event_bar_inner
{
background-color: green;
}
.complete70 .scheduler_green_event_bar_inner
{
background-color: orange;
}
.complete100 .scheduler_green_event_bar_inner
{
background-color: red;
}