Robert Oster Signature Sydney Lavender - colour variation on paper & on envelope.jpeg

Just a photo to illustrate that Robert Oster Signature Ink ‘Sydney Lavender’ varies in colour on different types of paper.

The upper part of this image shows the ink written on a piece of inkjet(?) paper (the back of a letter that I received).
The lower part of the image shows the same ink, from the same pen, written at the same time, on the envelope in which said letter was enclosed.