The heart of the problem is when you copy that white text from Mail to the clipboard, it is rich text (RTF), and apple assigns the color white to that text string. If Pages had the means to paste that text as UTF-8 text, instead of RTF, then the color information would be stripped and you would receive ordinary black text on paste.
To fix this in the specific case where you are copying white text from a black background, you can run the following AppleScript immediately afterward, and it will change the RTF content on the clipboard to just UTF-8 text. A subsequent paste into Pages will deposit black, not white text.
-- replace RTF text with white color on the clipboard with UTF-8 text
-- run immediately after copying white text from a black background
-use scripting additions
do shell script "pbpaste -Prefer txt | pbcopy"
return
- From Dock : Launchpad : Other, click on Script Editor.
- copy and paste the code from above into the Script Editor
- Click the Compile button
- File menu : Save…
- File Format: Text
- Save As: wb.applescript
- [Desktop]
- Save
- option key + File menu : Save As…
- File format: application
- Save As: wb.app
- [Desktop]
- Save
- Quit Script Editor
Now, all you have to do is double-click wb.app on your Desktop after you copy white text from a black background, and then a normal paste (of black text) into Pages.