Sorry for the delayed response. As per the issue description that you want to change the color of the E-signature. In Acrobat Reader mobile application there are some limitations regarding the E-signature. You cannot change the color or font.
But you do get the feature to change the color of the signature on the Acrobat Reader DC (freeware) which is for desktop.
Here is a help document you can refer to know how to sign a document and change color: