Four quick tips for those using watercolours:

• Write the colour number on the pan with a Sharpie (or a similar) pen. Inevitably the sticker or stamp with the watercolour’s number will fade away or peel off and you’ll be left trying to guess what it was when it’s time to replace it. If you mix watercolours by different makers then also give an indication of the maker on the pan.
• Replace unused paint with something else that you may actually use. This is important particularly if you’re working with a small portable palette. If you see a pan with paint that hasn’t got a dent in it, consider replacing it with something else that you may use. Ask yourself why it’s just sitting there (it’s too opaque and hard to mix? You don’t need it much because of the subjects you draw?), and replace it with something that will better fit your needs. Palettes evolve with time, and yours should evolve to better fit your needs and drawing goals.
• Buy tubes of paint to refill well used paint pans. When your starting out and still building up your palette and skills, work with pre-filled half-pans (the small square pans of paint). After a while you’ll start to see a few colours that you use more heavily than others, and for those paints you can either go for full pans, or buy paint tubes and refill them, giving them some time to set. It’s much more economical, with the caveat that you need to look up the paint maker that you’re using to see if there’s any difference between the paint in the pans and the paint in tubes. I use Schminke Horadam and there is none, so I’ve been refilling my pans for a few years now.
• Check transparency and staining before selecting a paint. This is particularly important if you plan on mixing paint or lifting paint (using a brush to remove paint from the paper while it’s still wet or flooding the paper to remove paint). Paint that is opaque will not mix well with other paints (you’ll get a muddy effect), and you need to be aware of it while drawing. Transparent paints may not be as vibrant as their opaque counterparts, but they work very well with others. Staining paints will stain your paper, leaving a shadow of themselves as you try to lift them. These are not bad things in and of themselves, these are just things that you need to be aware about before adding a paint to your palette.