Having taught art for a few years now, there are some common problems that I keep seeing so I thought I would share them and offer my advice on how they can be fixed.
Colours are dull and muddy
This is usually caused by the overuse of brown tones like burnt sienna and ochres. Another contributing factor might be not having clean water for brushes so every colour is being muddied. This is a common problem with watercolour, where colours can very readily become muddy. I have also seen learners not allowing an under-layer of paint to dry so that the colour layered on top mixes into it, which is not always the desired result. One way to solve this issue is to mix your own browns from the three primary colours (red, yellow and blue) and mix a range of neutral tones rather than using neutral colours straight from the tubes. Use browns with some caution, like black it is a colour that can make things look dull very quickly.