"Ravens Watching" 12x16, oil - $250 - contact Michael
The eye sees warm colors as brighter than they are and cool colors, darker than they are. When you try to paint the sunlit boughs of trees or sunspots among cast shadows, you will almost inevitably paint them brighter than they should be. If you do this, you will hit the upper end of your value scale too soon. You won't be able to paint the lightest accents, because you have already used them.
One way of getting a sunny effect right is to focus more on temperature than value. (See the above painting as an example.) That is, paint the warm sunny spots not so light but a little warmer, or the cool shadows a little cooler. Sometimes a temperature contrast will be more effective than a value contrast in creating the effect of sunshine. A mid-light yellow beside a mid-dark violet will "feel" a little more contrasty and sunnier than white beside black or a lighter violet beside a darker violet.