It is important for watercolorists to understand the difference between transparent and opaque watercolours

Opaque watercolours, contain more pigment and less binder, creating solid, non-transparent layers that can cover underlying colours. Transparent watercolours allow light to pass through, revealing underlying layers and creating luminous effects. They rely on layering (glazing) for depth, whereas opaque colours are used for bold, flat coverage.

Here is a list of colours divided into their transparent or
opaque quality. I have marked with an * the one that I
think would make up a good starter kit. Of course you
can add or delete according to your budget
.
Transparent Watercolours

• Green gold
• Transparent Pyrrole Orange
• Quinacridone Gold
• Quinacridone Magenta
• Manganese Blue
  Nickel Azo Yellow
  Quinacridone red
  Permanent Alizarin Crimson
  Transparent red oxide
  Se