How the Black Garment and Colored Addition Displays Work:
 
Bands - (Knit Mode) When a band is added, the black garment backs off by the width of the band, and the edge of the band is displayed in blue. Bands equal ribbings in knitting.
 
Facings - (Sew Mode) When a facing is added, the black garment line remains the same, and the facing width is indicated by a blue line.
 
Extensions - When an extension is added (e.g. in Center Front), the original center front line becomes dotted, and the extended line appears in black to denote the outer edge of the garment.

Hem - When you add a hem, the original hemline becomes dotted and the extended hem edge appears in solid black.
 
Seam Allowance - (Sew Mode) When seam allowances are applied, the original garment finished edge remains black, and the seam allowance edge becomes purple.
 
Quick Reference
Black Lines:  indicate the finished edge of the garment/pattern piece.
Blue Lines:  facings and bands
Dotted Lines:  extensions and hems
Purple Lines:  seam allowances