The color of the text on spruce, birch, jungle, acacia, bamboo, and partially mangrove signs are all colored far too dark when compared to the other sign textures. Almost all other textures use a darker shade of the sign texture for the text, making a much more natural texture.

Highlighting the text colors in Aseprite, you can see that the signs all use the same exact text color, which does not take the sign color into account at all:

Here is a comparison of how they look, vs. how one would expect them to look (keeping readability in mind)

Notes:

Dark oak is a special exception to text colors being mismatched, because putting dark text on the sign texture would make it flat and not legible.