When your font color for replies changes as the number of replies to an e-mail increase, you have Outlook set to pick a new color with each reply (this option requires Word as the editor or Outlook 2007). Go to Tools, Options, Mail Format, Stationery and Fonts to change it.

When you copy and paste text into the current reply and want it to match the current formatting, use the Paste Smart tag to change the pasted text to match destination formatting or use the Format painter to paint the preferred format over the newly pasted text.