In this example we will use the Sample - Superstore dataset.
First, create a line graph by dragging MONTH(Sales Order) onto Columns and SUM(Sales) onto Rows:
Navigate to the worksheet that you wish to copy the existing color from. Select 'Color', 'More Colors':
From here Copy the HTML code:

Now go back to the worksheet you wish to use the color on and select Select 'Color', 'More Colors':

Now all you need to do is Paste the HTML code in: