13 Jul 2018
If the creature is normally colorless, it will simply become black. It can’t be both black and colorless.
13 Jul 2018
A later effect that changes the affected creature’s colors will overwrite that part of Rise from the Grave’s effect; the creature will be only the new color. The same is true about an effect that changes the affected creature’s types or subtypes.
13 Jul 2018
Rise from the Grave doesn’t overwrite any previous colors or types. Rather, it adds another color and another subtype.