The resource pack attached to MC-214686 offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.

The bug

The torches on redstone repeaters and redstone comparators appear darker than expected, and are shaded depending on the view direction. This is not the case with redstone torches themselves, which are fully lit regardless of the view direction.

This began in 1.8, and was not the case in 1.7 or earlier. In addition, this is not the case in Bedrock Edition, effectively also making this a parity issue.

While this was listed in MC-68302, I still strongly consider this behaviour unintended for many reasons, including the two stated above. In addition, MC-67830's resolution contradicts such an assertion of this being intended. Other reasons are listed in MC-236474, a very similar issue.

How to reproduce

1. Place down a redstone torch, noting how bright it appears when in the "on" state
2. Place down a redstone repeater in a way that the torch powers it
3. Note that the torches on said redstone repeater are darker than the standalone torch
4. Now place down a comparator which also receives power
5. Note that it, too, has darker torches than one would expect

Expected behaviour

The torches on repeaters and comparators would appear as bright as redstone torches themselves

Actual behaviour

Repeater and comparator torches are noticeably darker

How to fix

The resource pack attached to MC-214686 fixes this issue completely.