If you apply two curve fits to the same Y variable, both of the curve fits will be drawn with the same line style and color. How do I have the curve fits use different colors or line styles?

Copy the Y data into a new column, plot both sets of Y data, and apply one fit to each. If you plot the data using a Line plot, you can hide the second Y variable by choosing None from the Show Markers pop-up menu and a Line Style of None (both in Variable Settings). The data will still be there, but only one set of markers will be displayed.