PowerPoint has no tool to partially fill a shape with a colour.You have to use a workaround to accomplish it.
Actually you can determine the percentage of the partial fill.
First draw any shape you want to fill, for example a circle.
In this example I would like to fill half of a circle with the black colour.
Select the shape and right-click it. Choose Format Shape.
By default the fill colour is Blue, Accent 1. In the taskpane first choose No fill.
Then choose the radio button next to Gradient fill.
By default there are 4 available stops and you will need them all.
Click on the first stop, click the Color button and choose White, Background 1.
Click on the second stop and again choose White, background 1. slide the stop till it gets to position 50%.
Select the third stop and click on the Color button, choose Black, text 1. Slide the stop till it is on top of the second one, precisely at 50%.
Finally select the fourth stop and apply the colour Black, text 1.
Since the type is set to Linear the fill will run horizontally.
The bottom half of the circle is now filled with the black colour. If you want the top half to be filled you have to choose 270° next to Angle.
If you want the fill to be vertical you have to choose 180° or 360° as the angle.
As an alternative you can use the Direction option.
Below some other examples of a 50 % fill with other shapes:
The same examples with a 75% fill:









