One thing that makes watercolor hard is gauging how dark (or light) a color is on the palette...before you put that virtually irreversible mark on the paper. This puddle of ultramarine blue appears darker than the yellow nearby but on paper they are pretty similar in value.
Rather than spending years of trial and error there are some things you can do to to get your mix just right (or close enough to successfully adjust it on the paper).
Sure, you can keep a test sheet nearby but its use is limited. You need to see a color in situ to judge for certain...that almost requires painting two pictures side by side. The best use for a test sheet is to make sure your initial light values are near or far enough from the white of the paper to provide the desired level of contrast for highlights or white passages.
Getting a feel for the paint's consistency is the trick.
First, tilt the palette a bit. Is the paint runny? Is it slow moving? Or does it stay put?
The images below are on a palette tilted about 15-20 degrees.
Loose, runny paint will make a pale to medium value mark (a lighter medium most of the time).
Slow moving paint makes a medium to dark-medium value mark. Note the deep end of the runny ultramarine puddle looks as dark as the paint that's moving slowly - not on the paper, though.
How it looks and feels on the brush
RUNNY PAINT will look and feel juicy in the brush tuft. The tuft will swell up and you'll feel the weight of it in a large, fully loaded brush. It will drip if you're not careful.
SLOW MOVING PAINT won't fill out the tuft as much but it will look moist. It rarely drips without tapping the brush.
PAINT THAT STAYS PUT looks drier and tends to be on the surface of the hair, not between the hairs from capillary action.
This should get you in the ballpark. Put a touch on the paper...still not just right?
IF ITS TOO DARK: quickly dip your brush tip into the water (don't dunk it) and bring it to the paint. You should be able to adjust value easily as you paint along.
IF ITS TOO LIGHT: immediately touch your brush into the color in question and bring it to the mark. If you're not dawdling the water in your paint will still be mostly on the paper's surface and it should blend in nicely.
Now you can see, there's something to be said for mixing a bit darker than you may think needed. For starters, it usually dries lighter anyway. Next, it's easier to bring water to lighten a mix accurately than to darken it by bringing additional paint...especially since your colors are usually a mix. Darkening a mix is tricky, especially when it's a medium to dark value. It usually leads to huge puddles of paint that don't get used and meanwhile your picture is drying up while you mix. All that leads to a condition called hydrodyschromo anxiety, a known trigger for other dysfunctional behaviors.
Another sly move is to mix your color darker deliberately and then put a bit more water on one side of the mix - not in the middle. Now you have range of values to dip into (great for green mixes when you often need a variety of values).
This all requires a few tries to be sure - but not the years of trying to get it by osmosis or something.