Sadly, the guide with the charts is now useless because attachment(pictures) died with the forum migration.

However, when it was up, I did do a stat gain per exp required calculation per level. And as it turns out, lower color have a slight better return at the same level. Purple leads gold by about half level to a full level, which means they should be kept at same level, with purple leveling first. Orange leads red about a full level, which means red should be a level lower than purple.
This does indeed all stop at lv 9, where you will stop leveling the runes until everything is 9 (purple first, then gold, then red). After that, you'd want to make purple 11, then gold, then red.