Find unused plot color

Find the first unused default color in a plot or generate a random one.

Whenever plots are generated inside a loop with the figure put on hold, the normal plot command plots everything in the same color. When a different color is wanted, normally a color-vector needs to be generated.
With this function it is possible to call the color inside the plot command, using
plot(x,y,'color', which_color(gca))

It first uses all the specified colors in the ColorOrder. If all of them are used it generates a random color.