Copying text from an outside source, like Microsoft Word or a website, can sometimes cause problems when pasting into CORE. If you need to copy and paste text into CORE, paste the text into Notepad first or a similar text program and then paste plain text from there into CORE. That eliminates any such issue.
Sometimes the text font can display in a different color when copying and pasting into the Compose Email screen. Select 'Paste as plain text', when pasting so that the font format doesn't copy from the source and doesn't get applied in your email.