mdColors directive will apply the theme-based color expression as RGBA CSS style values.
The format will be similar to our color defining in the scss files:
[?theme]-[palette]-[?hue]-[?opacity][opacity] - default is 1
?indicates optional parameter
mdColors directive will automatically watch for changes in the expression if it recognizes an interpolation
expression or a function. For performance options, you can use :: prefix to the md-colors expression
to indicate a one-time data binding.