Welcome to the forum, first of all!
And now to your scary encounter with Verve features, hehehe.
...but you'll see that this is totally normal.
If you have 0% buildup, it will not add new material, but paint color on existing material.
If your paint mode is additive (+), it will literally add the chosen color value to the existing color on your material, leading to a glowing effect as you go beyond 100% (1.0) of the color values.
Additive refers to color brightness being added, meaning that, for example, if RED on the canvas is at 80% and you paint with RED at 50% on it, it will immediately result in RED at 130%, but it accumulates as you keep painting to RED at 180%, 230%, 280% and so on...
Also, if you really want it to glow, hehe, you can use the little GLOW dial above the color wheel. That's good fun!
There are three paint modes: ( = ) ( > ) ( + ) ...which is ABSOLUTE (=), ACCUMULATIVE (>) and ADDITIVE (+)
If you want to just "add" paint like in the "normal world", you do not need additive paint mode. For that you use the ACCUMULATIVE mode (>). This will keep accumulating material on the canvas as you paint, creating really thick paint layers as you keep going. The build-up amount determines the amount of material on your brush.
If you want to paint more photoshop like (or traditionally digital) you use the ABSOLUTE mode (=). In this case it will use the build-up amount as the absolute amount of material that will end up on the canvas. Confusing there can be the fact that it will ignore what's on the canvas already and bring it to the chosen build-up level, looking like it's cutting into the paint, when you go over existing paint.
If you want to make things glow or brighten paint, you use the ADDITIVE mode (+). This one will deal with material like the ACCUMULATIVE mode, but color values will be added, as explained above.
I hope that helps?!