The first most significant component of watercolour painting is the "water."
Before you even start a painting, you need to be working to accommodate the properties of "water". Paper and water do not sit well together. When wet, paper swells and stretches, which manifests as rippling and buckling. Watercolour paper therefore needs to be prepared, before painting begins, to prevent buckling.
Preparation involves "stretching". Briefly, the paper must be soaked: 4 minutes total immersion is enough for a heavy weight paper. Next it must be stretched and secured to a support. Finally, it needs to be allowed to dry. As the paper dries, it shrinks back to its original size and, if properly secured, becomes taught and flat. This tension prevents further buckling when the paint is applied and the paper becomes wet again.
Paint "wetness" is a critical factor in painting with watercolour. Tint strength is controlled by moisture, and not the amount of pigment used. The wetter a color mix, the winner the tint will be. Generally, watery washes should be applied to wet paper. Areas to be washed should first be painted-over with clean water, and this helps the pigment wash to flow and disperse more evenly.
Conversely, strong color comes from a dryer mix of pigment, and dry mixes should normally be applied to dry paper. Dry mixes do not creep and flow, and the paint will remain exactly where you have applied it. Dry mixes generally have greater opacity, and are inherently more uniform. Areas of strong color can be softened at their edges, and this leads me to my next point.
The water solubility of the pigment is a critical factor in its manipulation. Water is used to carry the pigment to the areas of a painting where you want it to appear. Water can simply be used to dissolve and remove paint from places you do not want it to be. This makes corrections, such as softening and pigment removal possible. For example "bloom" can be removed by using a damp brush (Bloom is the pooling of pigment at the edge of a wash, which appears as a darker line).
The solubility of watercolour pigment can have incidental consequences. Application of a second wash over a first will inevitably remove some of the undering pigment. The trick here is to use this attribute to your advantage. For example, do not draw a layout sketch with a pencil; paint it with a pale mix of an appropriate color, and the sketch will disappear as color washes applied. A neutral colored (or even a plain water) wash can be used to retrospectively soften neighboring washes of different colors, since some of under coloring color will be dissolved and moved or removed (dependent on the wetness of your brush). Clean wet brushes move pigment: clean damp brushes remove pigment.
Most watercolour students are taught to work from light to dark: this acknowledgments pigment. However, there is also a need to work from wet to dry, and this acknowledges the water. The dry washes usually need to be done last, because a subsequent wet wash over a dry application will move and lift that paint.
The second most significant part of watercolour painting is the "colors".
Pigments are not simply different colors; they are chemically different have dissimilar characteristics in terms of their permanence, opacity, granularity and staining effect.
Reds and blues tend to be less permanent, and should be a little slightly overstated. After just a few weeks, they will mute down.
Some colors (some yellows for example) are more opaque than others, and this can influence color wash sequences. For instance, crimson over yellow can look different to yellow over crimson (but the end result will depend a lot on the make and quality of the paintings you are using). Cerulean blue is granular: expect to see little pools of deeper color within a wash.
All pigments work via a combination of staining and surface deposits. Some colors enter the fibers of the paper. They stain, and are impossible to completely remove by re-wetting and dabbing away. Others dry on the surface of the paper, and are easily removed.
I could no doubt write a list of the characteristics of common colors, but I think it should suffice to advise of the need to become aware of differences between pigments.
Although watercolour painting has a lot to do with mixing colors, tonal graduation, color depth and texture are achieved through controlling wetness: both the moisture of the paper, and the wetness of the paint.