Besides the postproduction options outlined in other answers, you can be intentional on how you use the elements instead of resorting to "fixing in post". You do have absolute control on the quality of the scene by careful crafting of the materials used and the lighting of the scene, without having to resort to additional effects.
If you want more ...
By default blender tweaks the final renders colors.
Go to the Render Properties tab (camera), and under Color Management, change View Transform from Filmic to Standard. Now the colors will come out nearly identical.
Blender's compositor has nodes that are analogous to controls commonly found in post-processing applications, and can separate color into several spaces. Some examples:
Blender's compositor is not very performant compared to other applications, but it can be made much better by checking a few options in the N-panel.
You can set the edit quality level for ...
I've worked it out, it's not an error after all. Once you select the Eyedropper, when you move the mouse the cursor will revert to a pointer, however the next click will change the color wherever you click in the Blender window. Just not immediately intuitive because the cursor doesn't remain as a pipette.
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.
PARTICLE MATERIAL (On sphere and cone)
EDIT - Here is a file with the Cube, as requested:
I think you have several ways to adjust the color balance :
In your 3D View, you can tweak the materials to make sure that they are as close as possible to what you want. For example if you mix your Diffuse with a bit of Emission you can make sure that you'll have colors that will less depend on your lighting (but of course the shadows will tend to ...