User blog comment:Atsuko-Regalia/Yumemi Nemu, Tone Rion V4, and Nemu Yumemi/@comment-53539-20170111100531

To me, I looked at the V3 design and noted the colour scheme, the V4 design is more or less a colour swap of the colours of most of her V3 design. Aside from the hair colour the face and hair are relatively the same, its just the different artist means the newer design is more smooth-edge and less cut-edge in terms of things like her hair.

So aside from the hair colour, the only big difference is the attaire, which is something that can change per release (see V3-V4 Flower).

The V3 design had its issues, it was basically a moe-check-list... Look up "cute" moe tropes and see if she has them. Fang? Check! Pointless bandaid? Check! Hair accessories? Check! Maid/school outfit? Check (maid)! Thigh high boots/stockings? Check! Etc, etc. The moe stuff was basically V3 Tone Rion was a lolicon character, and lolicon characters are meant to look cute-sexy... And her merch within the Vocaloid shop even reflected this. Note the only thing protecting her underwear being shown in the poster art of all the Vocaloids was the metal wing on her shoe. She was the only one whose positioning was like that, the rest was just standing there or doing something relatively normal.

The design was basically over cluttered and difficult for artists to draw in addition.

I do have a theory that the hair colour was changed because Nemu was created and the two now have colour schemes that contrast each other. One is pink, the other blue. So the theory goes, Nemu inherited the bluer hair colour and Rion's was changed to pink to match their colour schemes.