When you don’t stick to the manufacturer’s paint spread rate instructions you end up applying a thinner than required coat of paint, leading to visibility of the underlying color. When the underlying surface is dark colored and you are applying a lighter colored paint, you will obviously not be able to hide the dark color. Note that some colors such as oranges, reds and yellows have weaker hiding properties. Sometimes an uneven application by an inexperienced hand can cause this problem. Diluting the paint with an extra thinner to increase its quantity can change the opacity of the paint film, resulting in poor hiding. If the paint is not mixed thoroughly, it can show poor hiding capabilities at certain places.
Good quality paints are complete in their composition and do not require any further thinning. So avoid thinning your paint further. Mix a very small quantity of thinner only if required. Sometimes applying an extra coat of paint solves this problem. Make sure you use quality painting tools such as rollers with the recommended nap length. Also, use the recommended quality of brushes that help maintain the spread rate and give an even finish.
If you are using a lighter or an extreme color always use a primer tinted with the color you are about to use. Always use a primer on porous surfaces to avoid early drying and to maintain the spread rate.