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Adding my two-cents to the Opacity/Flow discussion...

As John Wheeler says, if you set Opacity to 50%, the brush strokes cumulatively give you 50%, then 75%, then, 87.5%, etc. It theoretically never reaches 100% no matter how many brush strokes you apply, but as a practical matter your eye can't really tell the difference once you reach 95% or so.  The mathematical term for this kind of thing is "Asymptote".  An asymptote is a mathematical curve that approaches a certain number without ever reaching it. An example would be the graph of the formula Y = 1 divided by X.  The greater X becomes, the closer Y gets to zero, but you never actually arrive at zero even if X = 50 trillion gazillion.

In Photoshop, the Flow setting is additive. I've never actually tested it, but if you set the Flow to 10% then each stroke adds 1/10th of the strength of the color and it should take 10 strokes to achieve the full color.  I use the Flow setting all the time for dodging & burning or to gradually apply a color or an adjustment layer. I'll set the flow to something very small like 5%, so each individual brush stroke gives you a barely perceptible change and you get a lot of control over the outcome.

Rich