Water colours are a transparent paint based on Gum arabic. Painting in water colour is a technique whereby the paint, thinned with water or as it is, is applied on special water colour paper. The paint has various constituents including pure pigments and Gum arabic, which ensures the pigments adhere to the paper. In contrast to gouache, water colours are always transparent.

– Talens

Water Colors come in tubes or in pans or half pans (this is the smaller size) The quality of the pigments is so remarkable, that very little paint is needed for filling even larger surfaces. Thus, in the last twenty years I never needed to get a refill for my half pans (and yes, I do make use of my watercolors) The only downside about this particular brand, that I can think of, is that it is rather expensive.

Unfortunately, when Nicolas and I moved to the States, we had to watch the weight of our luggage, and I had to leave my wooden box of exquisite watercolors behind – for the moment. In the meantime, I ordered a moderately priced Windsor & Newton Cotman Sketchers Pocket Box Set and am now working with twelve half pans instead of fifty, and two brushes. It is a decent set, but one definitely has to know how to mix colors in order to create more variety. In my opinion, having a larger palette to work with allows the artist to be somewhat lazy (I do not mean that in a derogatory way!) than if s/he paints with a smaller range of colors, because there is less mixing involved. A major downside of the Pocket Box Set is that it contains no black half pan. Instead, there is white – a color which is rather superfluous for this technique. Fate had it that in one line of the children’s poems, which I am currently illustrating, there appear mischievous gorillas which are, obviously, up to no good… It took me a while to mix a blackish tone for their coats: I mixed blues with browns, reds and greens. The result was not to my satisfaction (the coats just did not look dark enough), so, for the first time in this series, I tried a mixed technique; shading the coats with a black, extra soft oil base PITT Pastel Pencil by Faber Castell. I then dabbed the paint brush into some water and slightly smoothed away the noticeable pencil strokes. For a makeshift, the outcome of the gorillas’ coat is agreeable. However, it is slightly visible that an additional medium was used, which is why I will not apply this method again for this project. Apart from this, I have also changed the medium with which I normally draw the outlines of my subjects. Instead of a black fineliner, I now use a simple black ballpoint pen. The difference is obvious: While the fineliner looks “wet”, compact, and above all, very dark, the ballpoint pen is less thick and less dark, and the lines look “dry”. Nevertheless I prefer this medium because I can be more expressive with the lines, and I prefer the less compact look of the outlines. I will, therefore, continue to use it for my series (even though it is slightly irritating that this change in line structure is noticeable)

In conclusion, having to work with a different brand of watercolors when being in the middle of a project, proved to be less of a problem than was expected. It is hard to make a statement when, so far, I only produced one drawing with the different brand. However, I like to think that I noticed a difference in the pigmentation. Talen’s Rembrandt watercolors look slightly brighter, and are more transparent on the same paper than Windsor &Newton Cotman’s pigments. Then again, it is probable that I did not use the exact same range of colors, and therefore it is premature to make a final verdict.
Substituting a black pastel pencil for watercolors is, in my opinion, a good interim solution when one has to work with a limited palette, i.e., if the palette does not contain black, and if the painting/illustration is not part of a series of illustrations. (If it is, it might distract from the overall picture) Otherwise, it is best to buy the color separately (I am improvising and just ordered an ivory black half pan by Schmincke watercolors)

A good question and answer section about watercolors is in the bottom page of Blick art materials