Opacity

Opacity controls how much an object hides what is behind it. The same blue circle is transparent, partly transparent and opaque across the three panels.

At 0% opacity the circle is invisible. At 50%, the checkerboard shows through it. At 100%, the circle covers the checkerboard inside its boundary.

Changing the background changes the appearance of a partly transparent object. Judge overlays and text on their real backgrounds, rather than treating the blended result as one fixed color.