Usually, one responds to an existing post where an original poster has hopefully described their problem and seeks solution information. You have glossed over the requirements necessary to use the Pages Media Tool's Add Sketch, and that should be clarified:
- An iPhone 6S or later running iOS 13 or later
- Certain iPad models running iPadOS 13 or later
- Certain Mac models running macOS Catalina (10.15.*) or later
- The need for Bluetooth and Wi-Fi enabled on participating devices
The following links address the preceding items:
Continuity features and requirements for Apple devices - Apple Support
Use iPad or iPhone to sketch in or mark up Mac documents - Apple Support
To avoid this ritual in the future for the same signature, one can select and copy the captured sketch signature image once pasted into Pages to the clipboard. Launch Apple's Preview and choose File > New from Clipboard and save that PNG image so you can simply drag and drop it into future Pages documents that require that signature.
Or, one can click the Pages Image tab of the selected sketch and drag and drop the pasted-image.png icon outside of Pages to the Desktop or Pictures folder for future reuse.
One cannot alter the color of the signature once the sketch is deposited into Pages. That has to be done during the sketch on the iPhone.