I bought a pretty little curly leaf purple sage and now it is starting to turn green? What causes this?

I keep it inside and it gets morning sun thru the blinds. It appears healthy otherwise. I wait til it dries before watering, as I've learned most herbs don't like too much water.I bought a pretty little curly leaf purple sage and now it is starting to turn green? What causes this?
Probably not enough sunlight - herbs generally LURVE sun and variegated/coloured leaf plants will revert to green if they don't get enough.





Place in the sunniest site possible. Remove the green leaves, but, if ALL the leaves are green, remove only a third and, as new purple ones appear, remove the same number of green ones, until the plant is all purple again.