Cells color

Generally, the blue color of the mono and multi-cristalline cells results from the anti-reflecting covering normally used, but the choice of possible colors is wide. In the market, there are, in fact, cells in grey, yellow, red, green, silver and pink.

mc-Si colored cells sc-Si colored cells Thin film cells
Source: PVT-Austria, Sunways Source:  Solartec Source: VHF-Technologies

Consequences on power
Despite the fact that the colored cells produce less power ("standard" blue corresponds to an optimization of solar irradiation collection), aesthetic factors could become more important than power factors, if the available surface is big enough to cover needs.
An example of  performance of colored cells available in the market as compared to standard blue ones:

Color Multicrystalline cells (mc-Si) Monocrystalline cells (sc-Si)
Blue 100% 100%
Grey 73-80% 84%
Red 77-80% n.d.
Brown 81-90% 87%
Yellow 83-93% 81%
Green 89-98% n.d.
Magenta n.d. 78%