Callback: Decide object colour

Decide object colour (15B)

This callback allows you to set/modify the colour of an object upon construction. The lowest byte of variable 10 contains the current company colour or a random colour for when there is no company, i.e. in the scenario editor. If the object wants a 2CC colour mapping two nibbles, i.e. two colours, are passed in variable 10, otherwise one nibble (colour).