I use the Airlocks, typically in the smallest size. If I’m using a ton of weight (like several split shot along with the weighted flies), sometimes I’ll use the next size up, but I’d say 90% of the time I’m using an indicator, it’s the small ones. My most common setup is two fairly heavy nymphs and a single BB split shot. The little ones float this easily.
I used Thingamabobbers before the Airlocks and their performance was similar, but the Airlocks are gentler on the leader, and IMO are easier to make adjustments to, but that’s probably a personal preference thing. Just have to be careful about the screw on caps and washers. That’s the advantage of the Thingamabobbers, one piece design.
As with most things in FFing, pros and cons. I landed on the bobber style ones though simply as a matter of buoyancy and a desire to keep the indicator small. The classic bobber style ones just float better than yarn of a similar size. The foam style ones float decent enough, but they’re harder to see IMO and are just a total PITA to use and adjust.
Color, I don’t think it really matters that much. If the water is fairly clear I go white. If it’s off color, whatever the first one I grab out of my pack is.
Edit: As alluded to above, indicator nymphing is more of an early season/higher water tool for me. Once you get to low/clear Summer flows, dry/dropper (or just dries) is my go to, or if you want to stay completely subsurface strip the weight and indicators off and nymph old school.