Before you start mixing colors in watercolor, you have to get the raw tube pigment ready to mix. But just how much water do you add?
Unfortunately, there is no secret formula. It just depends on your style and what you're painting. There are as many ways to mix paints as there are artists. Here's a YouTube video where I show my pigment / water ratios. Every paint color you use needs slightly different proportions, so experimenting a lot on a scrap paper is a good idea. These are some of my most used paints.
I use a John Pike palette. I keep paints in these wells from painting to painting, keeping a squirt of pigment in all of them. I usually paint a few times a week now. Before kids it was rare for me not to paint every day. I keep a little less paint on my palette now than I did then, but I never skimp on paint. Skimping on paint leads to wimpy paintings.
How much paint I put in each well depends on how often I use a color and how strong it is. (i.e.: Azo Yellow is a weak color, I use it frequently, so I keep more in that well. Pthalo blue is intense, I keep just a dab of it.)
When I begin painting, I replenish the individual pigments that are low and spray water (about 1 squirt from a water mister) into each well. If I wait a minute, the pigment soaks up the water. If your watercolor paints use honey for their binder (I use M. Graham) then they reconstitute easily since they don't completely dry out unless you neglect painting for months. (Even I've been there!)
If you've ever tried to scrape up enough pigment for a shadow from a kid's paint set, you know what low quality paints are like! Most good quality paints will reconstitute immediately, regaining their brightness. Cheap paints are mostly filler or kaolin. Kaolin (clay) will not look great on your paintings. (It's surprising, but painting with mud will make your painting look muddy.) If paints don't mix back together, they are made cheaply no matter how much they cost. If they don't mix, don't buy that brand again!
Since most paints are lumpy, as I need an individual paint I dip in the well with a wet brush and stroke it into a puddle on my palette,making an intense pool of paint with no lumps in it. (Lumps are not the same as pigment grains.) Move your brush quickly in a small area and the lumps will quickly disappear. Watercolor paint dries quickly and some paints (i.e.: viridian) are known for getting flaky or lumpy even after mixed. So keep the real mixing until right before you need it!
I always mix paints as I use them, even tricky colors. If you mix up the perfect gray then splash it all over your painting, great! If you then paint the same gray all over your painting in the next 2 washes, you have a problem. You're making value changes but not varying the colors of each wash. Boring!