You can increase the number of contours using a fourth input, specifying either the number of lines or the values
contourf(x,y,M,50); % 50 contour lines
contourf(x,y,M, [-0.15:0.05:0.2]);
The latter option is better if you want uneven spacing of contours, for example higher resolution of lines near 0.
You might also be interested in this function: contourfcmap, which allows you to specify exactly which color you want to use for each interval.
