When you copy content from another source and paste it directly in the editor window, the editor will usually try to retain the pasted content's style properties (text colour, size, font, etc.). Typically this is undesirable, and even if it isn't we don't recommend pasting this way because it usually results in odd HTML code that could be inconsistent between user experiences.
The way we recommend pasting content from somewhere else is using the Paste as plain text feature. When you're in the editor, click the Paste as plain text icon above.
STEP 2: Paste your content in this new window (right click and select Paste). Then click OK.
You're all done! You can now continue as normal when creating content.
Note: If you're pasting something from Microsoft Word, you can alternatively use the Paste From Word button directly to the right of the Paste as plain text button. It works in the same fashion.
If you have any more questions, feel free to submit a support request here.