Colours cannot be combined in the same way they are with Offset and Digital output.
Colours should be solid. Gradients, shades and colour combining can be done using colour half-tones, however this is not recommended. This is a complex process that a designer will need to set up for you.
Ink colours can be "overprinted" on to each other – however as we cannot test every ink colour, the combined colour can be somewhat unknown, so it is not a recommended setup.