I created a particle system, duplicating a red sphere. However I want to also have yellow and orange spheres (already have them in another layer) in the same particle system. So I want to know how to use this particle system that I've created to show all three kinds of geometry/objects.
You will first need to select the objects you want to use, and press Ctrl+G to create a group.
Then use the Groups setting under Particle Systems > Render.
First, select the
Group render type:
Then select the group you want to use:
If you just want to have different colors you could also achieve this with the cycles materials. A node wiring like this picks randomly a color from a color ramp:
This way the number of particles having a distinct color can be easily controlled by the ramp settings.