Please help me on this issue. I drew a line using the polyline tool, and I want to leave part of the line black and I want to change only part of a line’s color. Please help? 
Thank you.
One way would be to cut the line where you want the line to change to another color then paint the section you want colored.
If you use the Ink tool (under the Paint tool) you can colour segments, that is, if you have lines crossing, the borders will prevent ink from spreading. This works with Pencil lines, such as the Polyline.
http://docs.toonboom.com/help/harmony-14/premium/colour/ink-line.html?Highlight=ink
For more detailed control there’s the Repaint Brush, which is basically a mode of the Brush tool that will only paint over pre-existing colours. Not to be confused with Repaint Tool under the Paint tool which works great if you just need to repaint everything with a selection or lines with a single colour.
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Luis Canau