There are no mistakes in painting. Only underpainting.
Sometimes placing your brush in the wrong color at the wrong time can be just the thing a painting needs. As artists we quickly discover our favorite colors and tend to come back to what we know best. But sometimes, its good to turn a painting on its head and reinvent it with an unexpected or 'wrong' color. Some of my best landscapes have come out of these happy accidents. I got distracted when placing the brush on the palette, slid one color into another, and slapped it onto the painting before I realized the error. With the new color in my landscape, the entire canvas needs to be re-evaluated, broken down, and brought up again. This doesn't always work out. But don't be afraid to keep working a painting after making a major 'error.' You might discover something wonderful.