Amateur do-it-yourselfers sometimes end up with a patchy finish on a new coat of paint; either the color painted appears in different shades across the wall, or another color seems to be showing underneath the paint. There is a tendency to panic in this situation, but it is still fixable; the solution depends on what went wrong.
Wait for the paint to dry overnight. Sometimes when paint starts to dry, it can look patchy as different areas dry at different speeds. It is possible that the patchiness will clear up overnight.
Apply another coat of paint. Even modern paints in dark colors require at least a second coat and commonly a third to get a deep, rich color.
Cover the roller in paint. Some people try to skimp on the paint and apply very thin coats. This may seem like an economical idea, but it does not work. Apply plenty of paint to the roller, then even it out across the entire wall -- do not apply a little paint and try to stretch it too far.
Cover the wall in primer and start over. One of the most common causes of patchy paint is not using primer or not allowing it to dry properly. Primer is a vital part of any painting job.