Pied Pearl Grey on the left, Royal Purple on the right. Cuteness times ten!
The key to telling the difference when they are this little are the head markings (broad dark stripe or broken/squiggly dark stripes) and whether the white on their wings goes all the way up to the shoulders or not.
My Pearl Greys carry the partially pearled gene (recessive) so I get Royal Purple keets out of them quite frequently. Royal Purples are basically a Pearl Grey bird that (due to having 2 partially pearled genes), have less pearling... if that makes sense.