Not always a good idea. To indicate with red ink, I have a lot of personal experience of having to ask people to sigh A MDA device that was always set to default "red" and in the area of London I was operating the North East Quadrant of London England; aproximately 30 mies in all directions I would occasionally come across people whose belief did not allow them to sign in red ink, it caused
a lot of distress and there were at least three occasions when I had to telephone my office and explain, that I was not going to ask the devout person to sign in "red". This involved waiting for my office to phone the client and explain whilst I had to wait discretely nearby.
I know there are one or two people on italki who do not care about such things that cause people distress, but I think it is a bad attitude to have. Italk askS us to be respectful of others. YoU can indicate with a different font or some other way.