The colorful plastic caps of the needles I use to lay a rainbow Dash mosaic every year (each year adding the needle caps I collected throughout the last year) do not allow for many different ponys for lack of color shades (It’s also why rainbows mane must remain without a dark blue streak of color here).  
There is however one pony extremely appropriate to be place alongside Rainbow and whose colors come close enough to what the needle caps provide. Apart from me being very much in favor of giving Rainbow Dash and Scootaloo more time together she may also help to reduce the future need for blue needle caps only in the mosaic expansions of upcomming years ;)