Start with an understanding of the paint products and brands you are using.  Work toward a better understanding of what to expect.    PLAY with different Brush strokes, try lifting a paint colour before it dries on your canvas, use the colour wheel to mix secondary colours.      Often students will ask 'BUT HOW DO YOU KNOW'                         how do you lighten a colour - this relates to understanding Value; and that each pigment has a natural value.                         how do you darken a colour without creating mud - this relates to understanding [...]