Two terms that sometimes cause confusion - and techniques that can save a painting from being dull.
Glazing - a term more usually applied to oil and acrylic painting, but sometimes to watercolour, in which case it would more normally be called a wash, is the application of a transparent colour over a (usually) lighter one. So, you might turn a bright yellow into an optical orange, just by glazing a transparent red over it. This will give you - in general - a rather livelier, more luminous orange than if you'd mixed red and yellow together.
In fact, most colours are more or less transparent, depending on the amount of water or medium you mix with them: so you can glaze even with an opaque colour like Cadmium Red - but you'll need to keep the paint to the minimum intensity necessary, and thin it down with medium. (Most manufacturers supply glazing mediums, in both oil and acrylic - Liquin is a popular one for use with oil paint, and Daler Rowney produce a good glazing medium for acrylic. Applying a lighter watercolour wash over a darker one will NOT work in the same way, or probably at all.)
Glazing also works if you add a transparent coat over a dark one - say, Raw Sienna over deep green, or mixes of red and green to give a "painter's black" (ie, a very dark tone, but one in which the use of actual black pigment has been avoided). This will enrich the paint surface, make it more interesting and subtle, and counteract the sometimes dead look that very dark paint can give.
Scumbling is different - it generally means the application of dry-brush (ie, there's very little moisture, whether oil or water, in the paint) over a flat, darker colour - in this case, the added colour is usually scrubbed on over the darker paint beneath. Again, the bottom layer doesn't have to be darker at all - the point is that you can see it through the added brushwork, and this adds depth.
Traditional oil painting techniques would have been all but impossible without glazing - it was a long process, especially since there could have been as many as 20 layers of glaze: and the paint over which they were applied had to be dry - otherwise, the paint would just mix. Acrylic paint makes this process much easier - in fact, acrylics are ideally suited to a glazing technique. Scumbled paint also needs to be applied on a dry layer, but adding multiple scumbles is rarely very satisfactory - at least over a fairly small area.
Trying one or both techniques in a painting can liven up and enrich an otherwise boring, featureless stretch, if dynamic brushwork alone won't do it.