I made a few custom models in my file and when I put multiple colours on them it creates several materials. Is this normal? Or like, is there a way to have two materials in this case?
It's normal, if you assembled boxes and cylinders provided with Sweet Home 3D, but you can change more than a material at a time in Materials modification dialog box, if you multiselect the materials you want to change with same color or texture.
Multiple selection in the materials list can be done with alt/option and shift key under macOS and Ctrl or Shift key under Windows and Linux.