What is the best media to use for adding colour to Textiles designs?

Do you mean designs on paper, rather than on textiles? If so, I'd be tempted to use a fine line black waterproof (Pentel do good ones, and available from Rymans and Smith's I think), and either keep them black and white, or use watercolour within the lines. If you do use watercolour, then use watercolour paper which won't buckle so much when you wet it. However, there's no reason why you shouldn't use a mixture, or even collage.