Snow! Real Magic adds variations of stairs, fences, etc that have snow on them, and when this is applied to grass stairs it changes the shade of the block in an odd way.

Forge version 36.1.0
Ornamental version 4.4
Undeco version 1.0.0
SnowRealMagic version 2.5.3

I suspect that Snow Real Magic is at fault with this. The discolouring is a result of the block colour tinting that Grass uses.

I submitted a bug report on their end as well but I figured I’d make you aware of it :)