Is the hydraulic fluid artificially colored?
A common hydraulic fluid used in aircraft is called Skydrol. The fluid is artificially colored, but this does not depend on which circuit it is in. The FAQ on the manufacturer's site addresses the color.


  Skydrol fluids are given a purple dye to make it easy to distinguish them from other fluids. But the dye does not usually last as long as the fluid does. The color may change from purple to gray or yellow or green, yet the fluid can still meet all used fluid specifications. Color is not a reliable indicator of fluid quality, and we recommend that the system be sampled for chemical analysis to determine if it is suitable for use. A dark amber color, however, is a cause for concern, and can indicate a severely stressed fluid. Call Solutia's Technical Service for assistance if you are unsure about a fluid's color. Our technical bulletin has a color chart to use as a guide.


Initially the color helps to distinguish from other fluids, and having multiple colors would make it more complicated. It notes that the color also does not remain constant, so even if it were colored initially, it would probably end up changing anyway. It also mentions that a dark amber color can be an indicator of issues.

I can't find any good pictures, but the lines are probably not color coded. Any colored hydraulic system components are generally purple (like the fluid). The lines themselves can be labeled, but the standard for hydraulic lines is blue and yellow. The end fittings are colored to identify the tube material type.

The maintenance manuals will show the layout and label the different lines. Also, on this ASRS page, one report is from a mechanic that confused the green and yellow system fittings, which probably wouldn't have happened if they were color coded.