After the first two of C-chan's drawings, he began drawing "chibi" OS-tan images in order to propose preconcept designs for a number of new OS-tan characters, without the labor-intensive delay of drawing full-body characters. Averaging about two chibis per weekend and with the designs being too simple to require much Photoshop post-production, the exercise steadily got him more and more accustomed with Inkscape.
Once he returned to creating another image with full-body characters, the drawing time was cut-down significantly.
The portraiture, featuring [once again] AmigaOS-tan with her older sister C64-tan posing for a picture, still shows some dated design elements, but at least is a bit more streamlined and dynamic than his first AmigaOS-tan attempt.
Amiga-tan continues to wear her boingball outfit, but C64-tan is wearing the undecorated tunic portion of her old military uniform (back when she was in the service of the old Commodore Family fleet). Presumably she wore a naval cap as well as a huge collection of badges and medals, but either lost most of those things or simply doesn't need so much "bling" on her present-day civilian self. The Naruto-style headband is not part of her clothing, but just a gimmick meant to show how up-to-date she [thinks she] is about Anime pop culture references -- with any luck, she's probably wearing a Code Geass prop by now.
Incidentally, the cap that Amiga-tan wears is also the last vestige of her old cadet uniform from the 1980's, which is why it features the now obsolete Amiga logo.
[Note: a pesky disclaimer is still in there somewhere, but is now at least better hidden.]