Assigning colour to QGIS vector layer fields?

I have a vector layer with a field called "dBM".

through query builder, I am hoping to change change the colour of the point depending on the value of "dBM".

e.g.

 "dBM" more than 20 ----> change colour of point to green

 "dBM" less than 20 ----> change colour to yellow

 "dBM" less than 0  ----> change colour to red