How to stop "additive" brushing

How well do you understand the Brush Engine and how the various controls work?  I'm going to assume that you are not 100%, so I apologise in advance if I am telling you things you already know.  Let's start by accepting that yoou had opacity and flow jitter set to zero, but the control is set to pen pressure.  I've always thought those settings could be confusing.

Jitter adds a random element to the brush stroke.  I've set brush spacing to a very loose 40% to better illustrate what is happening, so with opacity jitter set to 100%, you can see from the brush preview that each touch down of the brush has a random density — in this case ranging from 1% to 100% opacity)

Control has a loose association with jitter because it removes the random element, and gives us choices on how to control the setting.  You had it set to Pen Pressure, so the harder you pressed with the stylus the more opaque the stroke

You can see I started with a very light touch, and varied the pressure finishing on a lighter touch again.

I've never thought to combine pen pressure with random jitter, but it just means that when you apply a firm pen pressure the random element comes back into play.

The only other control I find useful is Fade.  The value controls each stamp of the brush, so the stroke goes from 100% to fully transparent in 1000 stamps of the brush.  Note I bought brush spacing down to 10% for this example.

For Build up, select a fully soft 100% opacity brush preset and using the mouse left click and hold down the button for a few seconds.  Now turn on Build Up and try that again.  With Build Up checked the stroke will continue to be applied becoming more dense until you release the mouse button.  It's the same with a tablet and pen.  Just touch down with the pen and hold it on the tablet.

Build up off (left) ............................Build Up on (right)

Now let's have a little bit of fun.

Lay down some random strokes on a new layer and go Edit > Define Brush Preset

Give about 50% size jitter and 100% angle jitter

Now give it 100% Foreground/Background jitter and 100% Hue Jitter

maybe add some scatter, but not much (I used 40%)

Make the foreground and background colours nice bright fully saturated colours

I decided to set size jitter to Pen Pressure

The best way to learn the brush engine is to play with it. It's a lot of fun and you'll discover all sorts of cool effects.

Same thing happens, see new screenshot: