After rasterizing my design, the colours look really bad. Why?

Reason:

When you rasterize your design, you create “holes” in the colours of your image. The material colour will show through these “holes” and mix optically with the toner colour.

The human eye blends these two colours together to form a mean value. In order to achieve better colours, it is advisable to increase the brightness of the design before rasterizing it. This will counteract this degradation of colour and may lead to your desired look. The brighter your image is, the smaller the dots will be.