I just re-read an article from Watercolor Artist magazine from 2010. It was called Watercolor Essentials - Tips to Achieving Luminous Colour by Rose Edin. I really liked how her colours popped and how well her paintings held together. Here's the points from the article that caught my attention.
Color Harmony - Rose paints with transparent pigments applying them directly from the tube to pape, allowing them to mingle or applies layers only after previous layers were dry.
Color Complements - Contain all 3 primary colours and enhance one another when they are next to each other.
Analagous Colors - Choose the pigment representing local colour as your primary hue. Then choose the colour beside it that is the lightest for the light values and the other on the other side as the darker shadow values. The warmer of the analagous colours will come forward in the painting and the darker will recede.
Mingling for Vibrant Color - Mingling creates exciting areas of colour instead of flat passages that are less interest to viewers. For proper mingling, use colours that are of a juicy consitancy.
Color Temperature - One colour temperature should dominate in your painting. Use the contrasting colour temperature to draw attention to an area of your painting; likely the centre of interest.
Harmonious Grays - Paintings should have some quiet muted grays to let the viewer rest his/her eyes. Complementary colours create greys or neutrals. You can control the temperature of the grey by adjusting the ratio of complements.
Layering - By layering one can darken hues or create vibrant new ones and thus play up or tone down areas. Build layers slowly so as not to obscure the underlying hue. It's a good idea to test layered colour combinations on a scap piece of paper so you know what to expect.
I applied the idea of analagous colours in a recent painting of a couple dancing. The dress was painted using analagous colours for the light, local and shadow colours in the fabric. This created a very nice effect. I used Quin Rose for the local colour, Quin magenta for the shadows and opera for the light values. I was pleased with the result.
You may wish to check out Rose Edin's website. I really love the vibrant watercolours she creates. She certainly practices what she preaches. If you have access to the 2010 article in the Watercolor Artist magazine, it is a qucik, informative read with great watercolour painting examples. I found it a good review.