mix(#ff6600, #666666, 70%)
for the last parameter 0% will result in the first color we stated (which in this case ff6600)
while 100% will do the 666666.
so in your case mix(color1, color2, 70%)
the output is 70% of color2 plus 30% of color1.
the default is 50% if you leave the parameter empty!