This post may look like a duplicate of MC-33304 but is, rather, "related"
REASON: this ticket was closed due to (on my opinion) incorrect reasoning. (Please read this ticket thoroughly!)
Please reconsider fixing/looking into this bug, (putting it on the low-medium priority bug-fix list?)

FACTS:

• It is not about the color of the sign, but about the texture of the sign! (IMPORTANT)
• It may seem to affect colors, but as seen in the attachments, it's just an illusion.
• Because of how lighting works in minecraft, the shading on the sign makes the sign look lighter/darker depending on rotation around the Y-axis; pointing one in X or Z direction will alter the sign's texture, NOT the colors.

Steps to reproduce
1: place sign facing X and Z direction.
2: coloring is only useful to see the major difference in shading.

using resourcepack:

• Can be seen that colors are fine.
• Because Black (Color #000000) does not alter it's shading, due to it being the most dark color.
• Changes in color are about #4F4124 -> #766034, which is a "color difference" of {Red: (dec) 39, Green: (dec) 31, Blue: (dec) 16}

  which is on average about (dec) 29 difference in color => 11.35% color difference.
  (found on upper right most pixel of sign)

[math might not be accurate]

Way to fix:

• Possibly: involves rewriting (parts) of lighting/shading system.
• Alternatively: Changing the way signs are rendered.
• Or some other way.

Comment on Attachments
2016-02-11_21.04.19.png = (no Res-pack) Viewed in line with Z-axis
2016-02-11_21.04.33.png = (no Res-pack) Viewed in line with X-axis
2016-02-11_21.21.18.png = (Res-pack: sign texture completely black) Proof that colors aren't changing, average color difference of maybe 1 or 2 (hex)(0.5%)
2016-02-11_21.21.47.png = (no Res-pack) colors "seem" to change, but actually it is the texture of the sign. (use a color picking tool for proof)

Final words and notes:
The coloring/formatting system, may not be fully supported, but texture/rendering bugs/errors are. The unreadability of the colored signs is just a side effect of the texture/rendering being darker/lighter. Black text is also harder to read, but since the contrast is so high, nobody will have problems with it. Unless the texture was darker, in which case you may or may not be able to read the text.