how can I make the material on the surface different color than the sprinkles on this cube? EDIT
I have played around with colors in Blender and I have yet a problem:
how can I make the background in different color than the sprinkles
on the cube ? Say, I want to make the background in red and sprinkles
in random green as they presently are. What shall I touch ? Please see the first snippet. I've almost succeeded with shader but the background is still green.

I have a wonder what to touch so that the cube in the snippet below becomes blue.
I'm new to blender.
To change the color, you must use the Material Properties tab:

To answer your edit, if you want the cube to have a different color than the sprinkles, you have to use different materials for the Cube and Particle Instance objects, respectively.
CUBE MATERIAL

PARTICLE MATERIAL (On sphere and cone)

EDIT - Here is a file with the Cube, as requested: