When I add a color to a fit line in the graph, the line through the associated legend symbol remains unchanged (Mac).

Prism graphs can include a legend. This is a symbol (or bar) followed by text that identifies the data set. When a data set is analyzed and a curve fit line is applied, a small bar will automatically appear through the legend symbol to represent both the data set and the associated curve fit line. You can choose, in the bottom right of Format Graph, whether to show just the symbol, or the symbol with line.

Changing the color of the fit curve should also automatically apply the same color to the associated line in the legend  but it doesn’t.

The color of the legend line should come from the curve but instead comes from the setting for what the color of the point-to-point line would have been. This is a bug found when using Prism 6.0c for Mac (and not Prism WIndows). It will be fixed in 6.0d. 

Note: There is no problem with point-to-point (connecting) lines, only with fit curves.