An eggplant is a very dark purple, with a red bias rather than a blue. I'd start with a dark red and add a some dark blue to it to mix a dark purple. Then for the shadow areas also try a black to which you've added a light red.
A red onion is a dark red that's got a purple bias, so I'd start with a dark red and add small quantities of blue to change it towards the purple side of red.
In terms of specific FolkArt® paints, I'd first try mixing with the colors you already have. If you want to buy additional colors, looking at the FolkArt's pinks color chart, I'd say 635 Fuchsia and 689 Pure Magenta and possibly 434 Berry Wine would likely be good starting colors. Adding some red, white and black or blue to these should give you a full range of colors for both an eggplant and a red onion.
