I have painted with Cotman, quite a few of the lessons in my gallery were painted with Cotman. The only difference I found between them and the artist W&N was the Cotman colours were not as vibrant.You may have to play about with them a bit more to acquire the colour you're after.... but it can be done. It's just a matter of varying the mix a little, one way or the other.
I am certain Val is correct. Student grade are probably just not as concentrated/intense as artist paint.Maybe mine were to old, they were given to me. They were very 'faint' and what was very difficult for me was that they would separate on the palette almost as soon as I mixed them.. including some straight out of the tube.Then someone gave me a few left over tubes of W&N artist grade to try (because the artist had switch to another bran and never emptied those tubes; and did not want me to quit prematurely). I decided to accept the tubes and try one last time while being encourage to tgive WC a try by a few PB members. I used those to do the cosmos and was very pleased with the results. The N&W rubes are empty now and I had replaced them with artist grade but a different bran as explained before. So replacing them as you go is a good choice. It won't be so expensive if you buy one at the time.