Tap the table so that the gray bars show then tap the bar above (for a column) or to the left (for a row). A section of the bar will turn blue and the entire column will be surrounded by a selection rectangle. Their will be drag handles (blue dots) to the left and right. You can use these to expand the selection to multiple columns or rows, as desired. A small drag bar on the selected section of the bar will allow you to make the column narrower or wider.
Once you have the selection to your liking use the paintbrush tools as normal.