To apply a background color to a certain cell/cell range or change the current one,
- select a cell, a range of cells or the whole worksheet,
- open the Cell settings panel by
- tapping the
icon at the top toolbar, or
- tapping the Cell option in the pop-up menu,
- tap the
Fill Color option to open the color palette panel,
- select the necessary color on the Theme Colors or Standard Colors palette (swipe it up to see more colors),

To clear the background color of a certain cell, use the
No Fill icon on the Standard Colors palette.