How Color is Affected by Adjacent Colors
A color is affected by the color next to it. Try this: Draw four squares about 2″ on a side and within each, draw another square about 1″ on a side. Paint the middle square of each the same color, say red. For the surrounding squares, use yellow, green, blue and gray respectively. Notice that the red appears different against the yellow background as compared with the others.
Another suggestion would be to set up a simple still life. Look at the various shapes of light and shadow. Mix one color and put it on the canvas, then another. Don’t worry if the color doesn’t look the same on the canvas as it did on the palette. Continue until you have the whole canvas covered. Then go back and make the corrections you need. Chances are that when you change one color, you will also have to alter the surrounding colors. This is good practice for your landscape paintings.