How to change the scale of the bar in Choropleth

I’m drawing a picture showing the scores in different state.

The scores are mostly around 3-4, which makes the picture wholy red without constrast. Could anyone help me to change the munber of the bar, like the min is 3 instead of 1.5 with 0.3 for a color so that the picture will be more contrast like this one:

Thanks a lot for your help.

Try setting zmin and zmax:

Thanks a lot for your help~~