to add to camille's response, the roots above the media are green because of the chlorophyll in them. they can make food with the light the same way leaves can roots are tailored to their environment so the ones below are made without the choloroplasts in them since there's no use. hope it helps.
Yeap, I agree with everyone else. White roots on a phal are fine, just means they've not been exposed to light.
Firm roots are good, mushy hollow roots are bad. The colour can vary and even brown roots are fine if they are firm, often they are brown due to staining from the medium.