Few plants will thrive where shade is very dense, particularly when coupled with dry, impoverished soil (one only has to look under a conifer or laurel hedge). It is unfortunate that shade-tolerant plants, in many cases, are not very colourful in flower. However, there is some consolation in their attractive or interesting foliage. Plants for shade gather what light there is through large leaves that are rich in chlorophyll and, therefore, are often very green. Variegated plants are less successful in shade than in sun as they lack the quantities of chlorophyll needed.
The solution depends on what is causing the shade. If it is a building, consider painting walls a lighter colour, or adding a mirror in a courtyard to reflect the available light. One can also use light coloured materials for garden features.