Color map: Changing scale using pcolor

I am using pcolor to plot data. The code is written below and the image generated is also shown.

This scale does not show the trends properly. How can I change the scale so that the same scale (-15 to -50) has more colors included in it and the contrast is better on the graph?

Code

[freq_grid,rpm_grid] = meshgrid(freq,RPM100_mean);
figure
s=pcolor(freq_grid,rpm_grid, log(resp1)');
shading interp
colorbar

The three .mat files 'freq', 'RPM100_mean' and 'resp1' have been attached below if requried.

Thank you!

[Walter Roberson]

I would suggest experimenting with different colormaps, such as colormap(hsv)