This mini-lesson will help you with layering and the frustrations that come with watercolour when you don't get the timing right. It is VERY finicky at first. Your results will depend entirely on the products you use, the climate you're in, and how long you wait.
In saying that though - there is no right and wrong. It entirely depends on the results you are trying to achieve on a particular occasion.
FIRST: result you get when you allow a layer to completely dry before painting another over it / adding additional paint. This is also called glazing. You can use it for fun transparent layers or to darken and add dimension to layers. No bleeding occurs. Note: overwork the layer under it and you may reactivate it or accidentally lift the pigment.
SECOND: what happens when you wait too long to drop in extra colour. The middle of the square was damp but the edges had dried. I simply dropped colour in the centre with a vertical brush and it started to spread but dried very weird as parts of the previous layer had already dried around the edges.
THIRD: the result most of us want. A layer damp enough to softly absorb extra colour, creating a smooth gradation. The same technique was used as the SECOND square however the THIRD was still damp all throughout, preventing the additional paint and water from drying weird. Note: too much water on your brush will spread the paint and water too far. Control your colour with a semi-dry brush (dab excess water on your brush on paper towel before applying)
FORTH: a very wet box (...that sounds wrong as heck to say). If you add more water or pigment to part of a painting that is this wet, it will spread all over the place, flood your page and buckle your paper if you haven't taped it down. This isn't exactly an undesirable result, some people love it, and a very wet page can be great for painting large areas and ensuring it doesn't dry up too quick - but it's important to know what will happen when you add more paint or water. That baby is gonna spread like wildfire. If you didn't mean for it to be this flooded, soak up excess water with a dry / thirsty brush, or gently dab a corner of paper towel in the raised water blobs.
MORAL OF THE STORY: this shiz ain't eaaaasy. It takes time and practise and EXPERIMENTATION! Please don't get mad at yourself over undesirable results, there are literally a million reasons why it may have happened. Try to approach "mistakes" in your work with a problem solving mindset.
RECOMMENDATION: make a chart like this! Get to know your supplies. Begin to understand how long is long enough to let something dry. Remember climate and room temp will affect this. Get to know how the paper will look when it's in the right or "wrong" stages of drying. Experiment with adding colour using a fully loaded, semi-loaded and dry brush.
Stay curious paint pals!