Dark purple is blue and red together so if you use ultramarine (151) and then 192 – Indian Red, you would get dark purple. The only hiccup there is that indian red is a bit temperamental. It is a hard colour (as a lot of the reds are) and when you are putting two colours on like that, if you press too hard with the 192 it will start to crush the paper. I would suggest that you practice with a piece of spare pastel paper. Colin would use the blue first and then the red but it is always worth doing the other way round just to see which works best. The other colour that you could try is a 138 and 160. They are a violet colour and you try using a touch of that with these other two colours as well.
We’ve included a Colour Chart on the right for the Faber-Castell Pitt Pastel Pencil range which refers to the colours mentioned above and is Colin’s recommended make.