Connie Achilles found a very logical way to assign colors to trigrams. From bottom to top the lines have frequencies from low to high. For the "pigment" colors (yin lines) the RGB-spectrum, the lines are red-green-blue from bottom to top. For the "light' colors (yang lines) the CMYK-spectrum, the lines are cyan-magenta-yellow, from bottom to top. Pigments are usually made from "earth", like e.g. siena. The sons belong to earth, the daughters to heaven, together they are white, or rather colorless light.

So fire, which has the second line yin, is magenta. Or � because it has the bottom and top lines yang, it is red + blue, and those two together are magenta. Lake is yellow, because of the yin top line. Or it is red + green, which is also yellow.

Cyan, magenta and yellow together make white light: trigram with all lines yang.
Red, green and blue make black earth: trigram with all lines yin.

When you take colors which are more alive, straight from a 'rainbow', they look better.

 

Hierarchy of symbols or archetypes

The ultimate is Wu Ji, the great void, usually depicted as an empty circle. After that comes Tai Ji, the symbol of yang and yin turning around and eternally alternating.

Wu Ji

Tai Ji

			yang	yin
small yang			big yang	big yin	msmall yin
mountain son 3	water son 2	thunder son 1	heaven father	earth mother	wind daughter 1	fire daughter 2	lake daughter 3

This arrangement makes sense in regard to the colors: they are split in additive and subtractive colors. The additive colors refer to light and to heaven. Together they create white (white light). The subtractive ones to pigment and to earth, together they are black.

For the explanation of the colors: see the page by Connie Achilles. It seems it is not on the internet anymore, so I put it HERE.