COLOUR: MUD
One of the things painters often worry about  is getting muddy colours, dirty colours.
You will read about this danger in books and magazines: DANGER!! MUDDY COLOURS!!
The first thing to say is that this is partly a matter of opinion and taste;
Some artists like clean, fresh, vibrant colours, others like more earthy characterful colours.
                                                                                       Aubrey Philips top Hazel Soan lower.

So be careful not to be unduly influenced by what others say, look at different artists and get a sense of what you like.
NEVERTHELESS; there is truth in the warning, it is possibly to arrive at a stage when your painting has lost its sparkle, its liveliness. it is overworked and the colours look dull. This is more a problem in watercolour than other mediums for a number of reasons and it is easier to fix in opaque mEdiums like acrylic or oil paint. But you can still get dirty looking colours in any medium.
there are three main issues:
1. the colours you use, and especially the colours you mix.
2. the way you apply the paint
3. the number of colours you layer.

1. COLOUR
Some colours are prone to muddiness, the more dense colours, some browns all the cadmiums, yellow ochre in particular, ultramarine blue, sap green, and of course black. Light red is a very dense colour.These are not 'wrong' or 'bad'  colours, there are no colours you can't successfully use,  but there are  colours you need to be careful with

Some artists choose colours which are particularly transparent; permanent rose, lemon yellow, phthalo blue, viridian.   For some this is way too bright, (but they can of course  be subdued by careful mixing)
I usually say: learn to use whatever colours you have, but be aware of the colours which cause you problems. There is no bad colour, but if you identify the colours and especially their mixtures which cause you problems you can work on improving either by substituting other colours or using those problem colours in a better way.

MIXING

Some colours are fine on their own but when mixed cause problems, ultramarine is a good blue but it can react with other colours to make effects which you may like or not like, with brown in particular it can go grainy,  (granulate to use the term people seem to like), this can be a good effect for a moody sky, or it can lead to a dirty looking colour. Sap green is to me a nice green but mixed with too many other colours it can go dull. There is a point mixing two or three transparent colours is usually no problem mixing together two or three of the more opaque colours can be a problem, ( but not necessarily, try it and see for yourself)

Some colours are simple and some are more complex, so lemon yellow is a simple colour, it is really one colour, yellow ochre is a complex colour, it is yellow but it brownish yellow, so it has bit of red and blue in as well. kind of hidden colours, so when you mix a complex colour with other colours you are mixing more colours than you may think.  Mix two complex colours and you have more likelihood of getting a dirty looking mix. 

 SOME ARTISTS WILL SUBSTITUTE A CLEANER BUT SIMILAR COLOUR, SO RAW SIENNA IS SIMILAR TO YELLOW OCHRE BUT IT IS MORE TRANSPARENT, lemon yellow with a touch if permanent rose may give a more transparent substitute for cadmium yellow. If you have trouble with brown, I find Burnt Sienna to be a cleaner mixing brown than Burnt Umber, but some artists would avoid brown altogether and mix it from transparent colours. (But as ever, you will find good painters using browns very successfully).

YELLOW: this may seem surprising but , especially in watercolour, yellow on top of other colours can produce a dull effect. It  best not used as a glaze over other colours to try and brighten,  put yellow on first, then other colours . Of course this isn't a law, try it and see. 

2. The way you apply the paint, if you watch a video of an artist,especially a watercolourist, you may note that they often have a light touch, their brush sort of dances across the paper, they don't scrub in the paint, they don't keep dabbing it and messing with it, they put it on fluidly and leave it to dry before applying the next layer. Colours are not in themselves muddy it is how we mess them around keeping touching them, especially when they are beginning to dry. Try and paint fluidly and don't keep dabbing and prodding the paint.

3. LAYERS,this is particularly important for watercolour, in fact muddy colours in other mediums can readily be fixed by a cleaner layer of paint being added once the dull layer is dry (some artists even paint a dull underpainting so their brighter colours can sit on top, giving the painting more depth). Watercolour being essentially a transparent medium this is not so easy.

I think in a successful watercolour some areas may have only one layer (some artists always leave at least a little white paper untouched), some areas with two layers; fewer areas with 3 layers, and only a small amount 4 or more. The more layers the more likely you get a dull clogged up. 

NOW THIS IS IMPORTANT: A VERY EXPERIENCED WATERCOLOURIST WILL FIND WAYS OF CONTROLLING THIS AND BE ABLE TO BUILD UP MANY LAYERS WITHOUT TROUBLE, SO THIS ISN'T AN ABSOLUTE RULE. The great thing about art is you can't do much harm whatever you do, so feel free to experiment with how many layers you can build up, the best way to learn is to do it, and reflect on the results.

Try a few versions of a picture. 

1. using colours which may produce muddy effect: ultramarine burnt umber yellow ochre, cadmium yellow.

2. Now try it with phthalo blue or cobalt blue if you haven't got phthalo (remember this blue has at least 3 names: Intense Blue, Winsor Blue, all the same colour), burnt sienna and raw sienna, and lemon yellow.

3 Build up a dark painting using  earthly colour such as burnt umber plus a little ultramarine in one strong wash

Now try to get that dark in 3 or 4 layers to  contrast the effects. You may like the more layered one, there isn't really a right or wrong but learn to recognise the kind of colour effects which you like, identify what you want to improve.  

I PAINTED A MUDDY PAINTING, WHAT CAN I DO?

1. You can wash off some of the paint, best be bold really and run the whole painting under a tap, but if there is only one area of an otherwise nice painting you may want to just carefully wet that area with clean water, and dab, don't scrub, the area with kitchen roll. The surface colour will come off leaving a softer stain. let it dry and re paint.

2. Some artists reckon if an area has gone dull you can glaze it with a clean bright transparent colour. I am not sure about  this but give it a try. 

APPENDIX 1. MY EFFORT AT CREATING MUDDINESS.

Here are a pair of pairs! something I have always found difficult is painting 'wrong' on purpose, (I get it wrong readily when I am trying to paint right)  here I have tried to do the scenes on the left with fewer layers and lighter brushwork, the ones on the right I have tried to overwork. As I said at the beginning it is at least partly a matter of personal taste and opinion whether one pair is better.