While going through some of my Golden Dawn papers I came across a diagram of two interlaced hexagrams, which show how the colours of the Zodiac signs are obtained. I have reproduced this here so that others wondering why, for example, Scorpio is "green-blue" can see the pattern.
This diagram needs some explanation. The colours of the planets do not entirely match the Golden Dawn's system, either for the King or Queen Scales of either the planets themselves or the Sephiroth to which they are attributed. The problematic ones are Saturn (typically indigo, or if we take the colour of its Sephirah, black), Jupiter (typically violet, though blue is applicable to its Sephirah), Mercury (typically yellow, though orange is applicable to its Sephirah), and Sol (typically orange, though yellow is applicable to its Sephirah). These planetary colours appear largely drawn from the Greater Key of Solomon.
Once we appoint six signs to the planets (Mercury to Gemini, for example), we then have the basic colours in place. The remaining six signs are simply combinations of the two colours on either side. For example, Taurus is between Aries (red) and Gemini (orange), and thus its colour is red-orange.
There are some other names used for some of these combinations, such as indigo for Capricorn, but this is a combination of violet and blue. Likewise, crimson (or ultra-violet crimson) for Pisces is a combination of red and violet, while amber for Cancer is a combination of orange and yellow.
While the Golden Dawn has not done so, we could arguably call all the mixed colours by another name. For example, the red-orange of Taurus is peach. The yellow-green of Virgo is lime. The green-blue of Scorpio is aquamarine. However, it is easier to remember the colours as combinations of the two colours on either side than a more fancy name.
Most of these match very nicely, but not all appear to match the Golden Dawn's colour scales. Unless the colour scales in Regardie's book contain errors (which is certainly a possibility, and I have not had time to compare them to original documents yet), there is one sign that doesn't add up. Leo is yellow on this colour wheel, but greening-yellow in the King Scale. Everything else appears to largely match.
Please note that the colours displayed in the diagram may differ on your monitor or might not match your definition of the colour. It is provided for illustration only.