Why did van gogh feel it is not enough to paint white spots on a blue black background?

II found this quote. I think it answers your question. I think he was saying the white dots weren't enough detail. with how beautiful a starry sky looks, he wanted more color in the picture and wanted it to show detail and emotion.

“At present I absolutely want to paint a starry sky. It often seems to me that night is still more richly coloured than the day; having hues of the most intense violets, blues and greens. If only you pay attention to it you will see that certain stars are lemon-yellow, others pink or a green, blue and forget-me-not brilliance. And without my expatiating on this theme it is obvious that putting little white dots on the blue-black is not enough to paint a starry sky.”

the ingredients listed on a label are listed by amount. the ingredient there is the most of is listed first and it trickles down to the ingredient there is the least of.