colourmap = colormap( hAxes );
where hAxes is the axes handle. gca if you really must!
You can then interpolate it to make it bigger or smaller if you wish.
If it is a builtin colourmap though, for example 'Jet', you can just use
colourmap = jet( 1000 );
to get a 1000-element Jet colourmap (not that I am advising using Jet, it was just the one that came to mind and is most similar to the one you show).
