This post is made after filing this "bug". TLDR: setting the same color value in a 3D text and a UI text, produces non matching colors when the project is using linear space. It seems that 3D text and UI text adjust the vertex color of the shader to "tint" the text rendering. While, UI text excepts an sRGB color value, the 3D text does not (and thus doesn't convert it to linear scale if needed). This is far from intuitive for a 3D text user and is an implementation detail. So, i think that this behavior should either be documented or fixed.