how to change color when reply to mail and make it default

Mail > Preferences > Fonts&Colors

the bottom of the pane allows you to choose the colors for "quoted text" in a reply to an email. It may require that you answer using "rich text" not just "plain text". That is specified on:

Mail > Preferences > Composing | Responding ...

[√] Use the same message format as the original message (rich text or plain text)

[√] Quote the text of the original message

[√] Increase quote level