Colors are the one bag decision that has nothing to do with function and everything to do with whether you'll actually enjoy carrying it. Here's a quick way to think through it.
If you wear mostly neutrals
Black or off-white are the safest picks - they disappear into almost any outfit. Black hides marks better for daily use; off-white shows more but photographs cleaner and feels lighter for spring and summer.
If you want it to double as a travel bag
Khaki and gray both read a little more "travel neutral" and hide the inevitable scuffs of airports and rental cars better than lighter shades.
If you want a little more personality without going bold
Brown adds warmth without being a statement piece - it pairs easily with denim, olive, and most fall/winter palettes. Blue is the one true "color" in the lineup, a muted slate rather than anything bright, so it still plays well with a neutral wardrobe.
The honest answer
If you only own one, black or off-white will get the most consistent use. If you're buying two - which is where the bundle pricing actually makes sense - pairing one neutral (black or off-white) with one warmer tone (brown or khaki) covers the most ground.
Plexiana comes in six colors: off-white, black, brown, gray, blue, and khaki.