How Light Can You Go?

Early on, it's a great idea to make bars on paper of each of your colours, so you can see the different tints you can achieve.  Watercolours are mostly transparent, so the colour on your palette or box may be misleading

You can do a longish tonal bar as you did with each of your pencils. 
You can make your changes continuous, or pencil in boxes and make each tint separate.

It's a good idea I think, to start with the darkest colour you can make (pick up lots of pigment and add only a little water) and then, on your palette, keep adding more and more water, applying it to a box on your paper each time, until in your last mixture you can hardly see the colour at all.

These bars are quite short, really.  It's good to develop the skill of control over your colour.