Measure the shape. If it's a square, measure the base and the height. If it's a circle, measure the radius.
Do not worry about the shape being only partially full; concentrate on the entire shape.
Calculate the area. For example, a rectangle's area is the base multiplied by the height.
Figure out the shape of what is filled. For instance, the shaded area of a triangle half filled with color becomes a trapezoid. Maybe what was filled does not resemble one shape but can be broken down into two, such as two squares, or a triangle and a square.
Take the measurements of the filled area.
Calculate the area of the filled space using the proper formula for its shape. If the filled area looks like two shapes, calculate the are for those two shapes and add them together.
Find the two areas that you calculated, the entire shape and the filled area.
Subtract the filled area from the entire area.
Use this number as the area of the unfilled space. This could be how much of a wall you must paint, or the area of a floor for which you need carpet.