You need to modify the built-in ginput() function since they forgot to add this incredibly obvious and needed option of color. For example see my attached code where I changed the [0,0,0] to [1,1,1] (white). Go to line 283
crossHair(k) = uicontrol(fig, 'Style', 'text', 'Visible', 'off', 'Units', 'pixels', 'BackgroundColor', [1 1 1], 'HandleVisibility', 'off', 'HitTest', 'off');
and change the color from [1,1,1] to what you want. Those are the R, G, B values in a range of 0 to 1. Then in your code, be sure to call ginputWhite instead of ginput().
You could even make a cross hair by calling plot() instead of uicontrol() and specify a dashed line if you want.