If you create the colourbar and keep its handle, e.g
hColourbar = colorbar( hAxes );
then you can edit the properties of the colourbar, in particular the 'Ticks' property. Or you can just do it all in one go if you knnow beforehand:
colorbar( hAxes, 'Ticks', -140:2:40 )
although I imagine ticks every 2 samples would look a mess with the labels. Every 5 probably ok.