When selecting the color for a line in a chart, I see 80 choices of colors. Is there a way to add new colors or set a color using RGB, HSV or HTML colors?
Not natively, however here is a workaround:
You can create a chart with the custom colors using Microsoft Excel, and then save this document and import it into Google Drive. If you then open the xlsx file with Google Sheets it will automatically convert the formats, and keep the custom colors you added.
This obviously limits the complexity of the charts and documents this is useful for, but I've found it helpful for creating a 'template' google sheet with the charts formatted the way I want.