1) In the image above, a cauliflower or backrun has developed because you have added wet paint to a drying area. (Although there may be occasions when you want this effect or texture – eg. in foliage or a leaf or petal edge – it would be a problem in certain paintings, e.g of a blue sky or a face!)
However, to correct the offending cauliflower, dry it completely with a hairdryer to limit the damage and then :
2) Lift out colour by “polishing” with a small natural sponge, dampened and wrung out, followed by dabbing with kitchen paper. This leaves a soft-edged paler patch which can stand as a highlight or even a cloud ( if it is in a sky.)
You can then paint over when dry. Obviously this is easier in a painting with lots of texture, e.g foliage, garden flowers… in a blue sky, you may need to add clouds by using your lifted-out area as a cloud and then add others as below:
3) You can lift out specific highlights with a damp ( not wet) flat watercolour brush, about ¼” across, used on its edge, followed by dabbing with kitchen paper. This could lift out masts, poles, tree trunks and branches etc – in other words clean straightish lines – or you could lift out specific shapes, even using a cut paper mask of the shape you want, like the moon, the sun or a boat.
4) You can always darken areas to mid-tone or darker by simply painting over already dry paint; you don’t need to ( and shouldn’t) cover the whole of the paler area. For a softer edge or smoother finish you can dampen previously painted areas of paper before you paint.
5) Watercolours can be given more punch and drama if you use a black fine pen to draw in details or darker areas.
6)You can use white acrylic paint or white gouache to restore lost highlights or to reintroduce light areas. If you want these tinted you can mix watercolour with the white acrylic paint to create a pale tint e.g golden, blue, yellow, pink, lilac.. if you want a pale green highlight remember to mix yellow into your green as simply adding white to green makes it very cool and ghostly.
Similarly if you want a highlight on a red poppy or red dress, or red lips, don’t just add white as you’ll get an ashy strawberry pink – add a touch of Cadmium Yellow to this to create a gorgeous coral colour.
7) If your watercolour lacks punch you can improve it using pastels in places over the top. Use either soft pastels or oil pastels, to add deeper or brighter colour touches. This can also add texture, e.g to stones or a stone wall, a beach, the trunk of a tree, foliage…
So never believe the myth that you cannot correct in watercolour! In all paintings for sale online and in galleries, either in watercolour, oils, acrylics, pastel, there will have been corrections / developments as the work progressed. You CAN correct in watercolour!