Spectrogram plot not filling out colours

I'm trying to plot a spectrogram, and I'm following the examples in the documentation, but I keep getting something that looks like this:

It's almost as though the color samples aren't filling out the plot. In the examples the spectrogram is solid colour, not with white. How can I make it work like that?

This is my code by the way:

sound = wavread('H:\Tin Whistle\Recordings Session 2\Key - G\G1.wav');
x = (sound(:, 1) + sound(:, 2))./2;
Fs = 44100;

segmentLength = round(numel(x)/4.5);
spectrogram(x,round(segmentLength/5),[],[],Fs,'yaxis')

Oh also, there's a color bar in the examples, how do I get that to display?