Well, the basic rule is that the farther the vowel away from the stressed syllable the more dimmed it is. I mean the closer to the shwa. A perfect example is the "молоко" word. You don't say it "малако", it's said like [мəлако]. The 1st О is almost, like 99% is a shwa because it's relatively far away from the stressed vowel and the second O is like 70% of A and 30% of shwa because it's closer to the stress and vowels near the stressed syllable become clearer.