I've seen these little balls in more than one brand of potting mix / universal compost. I just sifted a large amount of compost and found many.
I've seen them in the following colors: white, green, orange and yellow.
At first, I thought this would be slow release fertilizer, but I'm pretty sure they are not.
They literally explode when you press them with your fingers. There some liquid inside. There were no bugs/insects in the soil. I don't think these are insect eggs, also.
I'll try to photograph them under the microscope and will add it here later.
What are those?
UPDATE: Images with microscope