Is it possible to change the Trend line color?

I feel like I've done this before, but I can't seem to get it to work. 

I would like my trend line to be a different colour from my chart, however, when I change the trend line colour, nothing happens. And changing the colour of the chart changes the colour of both the chart and the trend line. In the screenshot you can see the colour I've chosen for the trend line is red, but it is not updating.