Limit the precision for custom colors

Using full double precision for storing colors makes no sense.

Truncating to 6 decimals yields a 60 bit (3 (RGB)  * 20 bit and 2^20 ~ 10^6)
color space while very high-end applications only use a 48 bit color space,
which would be about 5 digits.

E.g. regular Photoshop CS6 supports up to 32 bit RGB color spaces, so
clearly, 60 bit is already way overkill, but it reduces the clutter by half
in the color definitions.