This error is logged when reading color.properties from John SMith Legacy - JimStoneCraft Edition:
'Error parsing minecraft:optifine/color.properties: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 664 path $.text.sign'

color.properties: https://gist.github.com/dexman545/1c47efca56009aefa084db1c285fa3b5

Alright, thanks!

Comment out the line for the text.sign=404040 (stick a # at the front) to remove the error, good as a temporary fix.

It looks like the custom sign text does not work.

I would say since Mojang added the different wood signs this feature probably doesn't even work in OptiFine.

I'll change it in the next release of the resource pack.