Through the years I found out that it's hard to avoid this. Especially with the layering technique I use. Here's an example:
|Sunset, 50 x 150 cm, oil on panel, underpainting|
Not very promising, huh?
In the following layers I slowly built up the colors to the level of saturation I wanted. Because I use quite a lot of medium it took a few layers to get there. This is how it turned out:
|Sunset, 50 x 150 cm, oil on panel, finished state|
Check out my website for a bigger picture: http://www.paintingskies.com/products/
As you can see the initial color of the sea in the underpainting was quite dark. I added the lighter accents in the next layers. This sometimes works better than the other way around.
Anyway, if you sometimes get the feeling that there's a wide gap between what you had in mind and the present state of your painting, remember that's how it's supposed to be. Just keep going and somewhere along the line things will take a turn for the better.