Click the color you want to use and you will see that the Current Color changes to that color. This is your drawing color.
Now there are two ways to draw. You can "paint" by clicking on a cell in the chart. That cell will turn the color you just selected. Move your mouse around the chart and you'll see that any cell you pass over also changes to that color. Now click again and the drawing will stop. To fill in a single cell, just double-click on that cell.
You can switch colors at any time, filling in cells until you like what you see, then click Save to save it.
Note: You can click save as often as you like. Actually you probably should. That way if the power goes out or you lose your internet connection, next time you come back whatever you saved will be here waiting for you.
Another way to draw is by row and by column. Click the number of the row or the column to turn the whole row (or column) to the selected color. You can also change every cell in the chart by clicking the 'o' in the top left hand corner of the chart. You'll get a pop up warning asking you to confirm that you want to wipe the whole chart. This is just to make sure you don't wipe your pattern by accident.