You pretty much just have to discover which are clear.
So far, the Vapor4Life WOW juices I have tried were clear.
WOWBacco (cocoa tobacco, and I assume WOWBoy which is the same + mint), WOW Vanilla, WOW Sour Cherry
And if you have a KR808D-1 or can get an adapter to 808 aka KR808D-1 for whatever you are using, you can try their carto sample packs to try many of the flavors and just look at the stuffing to see if it left residue.
I don't know about the juices the other KR8 only vendors sell (SmokelessImage, Bloogplanet)
FreedomsmokeUSA oRYental 4 (a sweet RY4) always seemed pretty clear, and Pomegranate is clear
And though I will not put 50/50 juices in cartos, I do drip Voodoo (clear) and Hopscotch which seem pretty clear
Gourmet Vapor vanillarum is pretty clear (and BTW SALE 50% off until Oct 14 / code vapetv). Note - if you use their Flavor Creator, I think the flavor state if they have colorings. Yes - I just checked - if you use the flavor creator (requires login) and start a new juice, and choose a flavoring for it, to the right it will list all ingredients of the flavoring. So you could play there during the sale - I just recommend everyone mark your own flavors private until you have tried them, and then if one comes out good you can go back and make it public.
Re DIY - same deal. In most cases, until you buy the flavoring you usually won't know if the flavoring is chock full of possibly particulate colorings/ingredients. I noticed HappyVaper in Canada does note on some DIY flavorings that they are dark so you could use that as one guide. Perfumer's Apprentice site - I think they list flavoring ingredients. Some of the ECF DIY Guide lists and linked lists note coloring ingredients.
http://www.e-cigarette-forum.com/forum/diy-e-liquid/