Why does it become a new color letter when I send a new text message on my iPhone?

If you set "Show Subject Bar" to ON in SMS settings and write content in the subject bar of SMS editing bar, it will be sent as a new MMS. You just need to set the "Display Subject Bar" to off in the information settings. Or when editing the short message content, write it on the bottom line of the subject bar, so that it will not become a new multimedia message to send.