I didn't say "it's just a drawing". You added the "just" which changes the meaning.
I said it is a drawing. It is not a bull. It is a drawing of a bull. They are not the same thing. There are a lot of degrees of freedom in a drawing (together with iconographical traditions).
The color is very similar to some of the plant roots. There's no reason to presume that the color of the bull is marginalia.
You keep calling it ink. But that brick color might be mineral-based pigment or a mixture of mineral-based pigment and gall ink (they frequently mixed their textual ink with other pigments).
The gall ink (the blackish brown and the watered down gall ink which is brownish or even light brown if it is very diluted) are different from pigments that are made from ground stone.
I said it is a drawing. It is not a bull. It is a drawing of a bull. They are not the same thing. There are a lot of degrees of freedom in a drawing (together with iconographical traditions).
The color is very similar to some of the plant roots. There's no reason to presume that the color of the bull is marginalia.
You keep calling it ink. But that brick color might be mineral-based pigment or a mixture of mineral-based pigment and gall ink (they frequently mixed their textual ink with other pigments).
The gall ink (the blackish brown and the watered down gall ink which is brownish or even light brown if it is very diluted) are different from pigments that are made from ground stone.