What is the proper way to create a mask?

It is a bit hard to be sure when the artwork is so complex, but this is what I think is going on.

The reason for the color difference is because you have background, fluid (transparent red) and foreground. The red goes darker when you combine the three (two transparent greys and transparent red).

I think you are putting a white rectangle as part of the artwork below your device, then sliding the fluid down behind this while shape. That is a way to do it *without* using masking. That should "just work". I *think* when you add the masking (from the menu) you are actually masking out a grey layer, not clipping the artwork, so it is just one layer of gray instead of two, so it looks lighter.

Two suggestions to try. First, just don't use the clipping mask feature from the menu. Just have a white rectangle to hide your fluid behind. Does that satisfy your need? If so, done! 😉

The alternative is to correct how you are using masking. I recommend creating a sublayer to put the fluid into. The clipping mask arrow icon thingy then goes onto the artwork INSIDE that group, not the container as you are doing it. You also do the Transform on the layer INSIDE the group. You then create a second layer inside the group the same shape as the fluid, but make it a solid color. E.g. copy the fluid layer, then make it solid (not opaque) green or similar. Then hide this layer. I am pretty sure (have not tried sorry) even when hidden it will still clip the fluid layer. The mask must not move. The mask must be after the fluid layer. The fluid and the mask layer must be the only layers in the same group. The clipping flag must only be on the fluid layer inside the group.

Here is a discussion on using masking for eyes. There is lots of irrelevant content, but the masking part might be useful. https://extra-ordinary.tv/2018/04/21/debugging-character-animator-eyess/