I was trying the 1st tutorial for zb4r2 on painting with materials . It looks like the teacher just chooses another material and starts painting . When I try it my model changes to the new material . No chance to paint with it .
you need to make sure you have either MRGB or M turned on when you’re painting the material so it knows to apply a material.
you also need to fill the object with a material first.
color.fill object
As beta_channel says (thanks beta_channel 
), you need to first fill your object with a base material by pressing Color>Fill Object while either M or Mrgb is turned on. Filling an object with a material removes the default behaviour where the model will only show the selected material. You can then paint the model with different materials as well as color.
:)thanks