Hello and Welcome!
Waterbase, even though not plastisol, will need a base white under it on dark colors to support the intensity of the ink. The waterbase is very thin and soaks up into the shirt color, thus disappearing. The wasterbase printeres here will have to chime in as to what you can do for waterbase printing that is not discharge. An easy option that many do, is to print a plastisol base white down first, flash it and then print the waterbase over top. There might be a waterbase (white) that can go down first, but I would assume you would need to semi flash that also. Just maybe not as much as plastisol. I donno about that one. I've never used a white waterbase.
If it's discharge waterbase, then it should be able to be bright yellow depending on the saturation into the shirt. You won't see this happen until you run it through the dryer (for a full length of time such as 1 min, and the temp the ink calls for. That's where the action happens. I hear the best dryers are gas dryers (hotter?) and I also hear that you need a long chamber for the ink to be curing for the proper length of time and temp.
I'm no expert, practically a noob myself at waterbase and discharge.