Sunburst checks what mana was actually spent to cast the spell. If an effect allows you to spend mana “as though it were” mana of any color or type, that allows you to spend mana you couldn’t otherwise spend, but it doesn’t change what mana you spent to cast the spell.

The five colors are white, blue, black, red, and green. Colorless is not a color.

If a spell has multiple instances of sunburst, each one applies. For example, if you activate the abilities of two Solar Arrays and then cast Selfcraft Mechan (an artifact creature), it will have two instances of sunburst and will thus enter with two +1/+1 counters on it for each color of mana spent to cast it.