Use 3 param method for converting r,g,b into css string.

This avoids creation of temporary arrays to pass them into the util
method. Also using "arguments" is more expensive then passing in 3
parameters.

FYI, here the benchmark which shows the cost of using 'arguments':
http://jsperf.com/arguments-or-explicit-use
Is there any reason for the double rounding here? We could just as well use
`var r = Math.round(255 * (0.3 * color[0] + 0.7));`