DPlot

Color of generated curve matches background color of plot or hasn't been assigned a visible color. If I go to hide curve, it shows and will let me hide the curve, with no corresponding color line displayed. I presume this means the curve exists.

DPlot version 2.2.6.5

On colors. If I copy a curve (curve 1) from a plot to a new document, then copy a curve (curve 2) to the same new document, the two curves should not be assigned the same color.

Color of generated curve matches background color of plot or hasn't been assigned a visible color. If I go to hide curve, it shows and will let me hide the curve, with no corresponding color line displayed. I presume this means the curve exists.

Check that you have either a line or symbol style (other than "None") assigned to that curve. Click the "Line/Symbol Styles" button on the toolbar, or select the Symbol/Lines Styles command on the Options menu.

On colors. If I copy a curve (curve 1) from a plot to a new document, then copy a curve (curve 2) to the same new document, the two curves should not be assigned the same color.

If the colors are the same it is because that's how the palette in the target document is set up. (Since in this case it is a new document, that means your default color palette has the same color assignments for curves 1 and 2.) If that's not what you want for the default, click the Colors button on the toolbar, make the changes you want, and check the "Make defaults" box.

I've never changed the default line style or assigned different colors to curves. Relatively new and didn't know feature existed.

I'll wait til it happens again and provide more detail

One other thing to check is that the curve in question has more than one data point. If there's only one data point AND you don't have a symbol style selected, nothing will be drawn.