As far as I can tell, this commit to DefinitelyTyped fixed this issue. For me, lineColor works both as lineColor='red' and lineColor={'red'}.
141 might be a duplicate, but it was posted in March (about a month after the interface update in DefinitelyTyped) so I'm not sure if the problem persists despite the interface update.
export interface VerticalTimelineProps { animate?: boolean | undefined; children?: React.ReactNode; className?: string | undefined; layout?: '1-column' | '2-columns' | undefined; }
The interface is missing the lineColor property, the function behind works fine.