By default, Graphmatica will rotate among the 16 graph colors in your chosen color scheme every time you enter a new equation. If you want to change the set of colors available to rotate through, use the Colors tab of the Graph Document Settings dialog box (Options, Graph Paper menu).
To change the color of a specific graph, right click anywhere on the curve and select a color in the Pick Graph Color context menu.
PICKING THE LINE STYLE OF A GRAPH
By default, Graphmatica draws most graphs using a solid pen. (The curves for strict inequalities are drawn using a dashed pen to indicate that the solution does not include the curve itself.)
To change the line style of a graph, right click anywhere on the curve and select among solid and three dashed/dotted line patterns in the Pick Line Style context menu.