This is a question people have asked me often and if you are wondering too, here are my tips to avoid muddy colours:
1. Use Artist Grade paints.
Artist-grade paints contain a higher amount of pigment while student grade paints have more fillers so they can give a muddy appearance quicker.
2. Try not to mix more than 3 pigments.
If you mix more than 3 pigments, chances are you will mix the 3 primaries and, when you do that, you're likely to end up with a muddy colour.
3. Try to use paints with only one pigment.
Following from point 2. if you use paints that already have more than one pigment you're more likely to mix muddy colours. For example if you mix a colour with 2 pigments and a colour with 3 pigments, you'll actually have 5 pigments in the mix
4. If you use pans, try to always keep the paint clean.
It's unlikely to dirty the paint in tubes, but if you use pans, contamination of colours is very easy. That's why I use two pans of the same yellow because, no matter how hard I try, I always end up with some other colour in the yellow. So I use one pan of yellow for mixing oranges and another pan of the same yellow for mixing greens.
5. Clean your brush properly.
Following form the previous point, if you keep your brush clean when switching from one colour to another, you will be less likely to transfer colour where it shouldn't be. Also, I use two pots of water, one to clean the brush and the other to dilute the paint or wet the paper.
6. Try to use transparent or semi transparent paints.
Opaque colours are more likely to give you a dull effect when they dry, so try to use transparent, or at the most, semi transparent paints. They will stay more brilliant and will shine trough layers.
All of the above is just my advise but are not hard rules. I actually have one paint (quinacridone gold) which is made of 3 pigments; but if you're just starting out it will be useful to know these little tricks to help you keep your paint luminous.
If you haven't watch it yet, I'd advise you to watch my Skillshare Class How To Mix Luminous Watercolors where I go into more detail about mixing luminous watercolors and avoiding muddy colors.
If you don't have a Skillshare membership, my link will give you access for one month free.
I hope you've found these tips useful and if you want you can also download my “Guide to Watercolor Basics and How to Choose Your Favourite Colors”where you will find even more information about watercolor.
If you have a specific question, please don't hesitate to leave a comment below or contact me :-)
Happy painting!