For sketching the Sun, I'd first suggest trying the colouring pencils you may have at hand. I've found that cheap pencils tend to be too hard, hence don't make much of a mark with light pressure. Much better quality pencils tend to be softer, making laying fine details easier, and more intense colouring needing less pressure to lay down, avoiding damaging the paper texture too much.
If you can't find a soft enough colouring pencil set, use a 'white charcoal pencil'. The stuff is as soft as chalk. You can do your sketch with this, then on the computer, you can change the colour of the white to what ever you like. This following ASOD sketch is a fine example:
http://www.asod.info/?p=2563
You can also use crayons to work on the white paper. Have a look at this following clever sketch. The solar details where laid down using a green crayon on white, and then the sketch was scanned and the colours inverted!
http://www.asod.info/?p=2868