• Mod version: Alpha 1.12_3.98
• Minecraft version: 1.12.2
• Forge version: 14.23.5.2838
  

I have architecture blocks darker than their parent blocks, but only with shaders. Without shaders everything's good. Does it depend of shaderpack? In the preview the blocks are normal with shaders.

The same problem appears without any mods except arhitecturecraft and any texture packs.

More examples. The degree of difference varies with the shader chosen (I use chocapic here, though it's also noticable with wisdom shaders). It's definitely more noticeable with lighter coloured blocks; white wool becomes grey wool:

Same version everything as OP, by coincidence.

Architecturecraft does some weird stuff with rendering, honestly impressed it works at all with optifine let alone shaders.

Might end up working when I tear out all the rendering code and put some new stuff in. Not sure.

Anyway, optifine isn't supported. If you have rendering bugs with optifine and or shaders then I recommend you remove them.