Find the color palette on the left hand side of the screen, underneath the toolbar.
It has two boxes at the top - the primary color (top box) and secondary color (bottom box).
When you click on either of these color boxes it will show the color chooser where you can choose a color to use.
Select the 'Pencil' tool from the toolbox on the left.
With your primary color selected, click on the pixel you want to color. It will change to the color you've chosen. Now try drawing with the color you choose.
Experiment with different colors by changing the primary color in the palette.
Now let's use the secondary color box.
Click on the secondary color box and the color chooser will appear.
Choose another color and this will save into the secondary color box. You can click the arrows to switch between the primary and secondary colors.
Now that you know how to choose different colors, create a colorful sprite and try out your new skills!