参考了三份有用的资料：

1.关于richtextbox设置字体颜色的问题

http://biancheng.dnbcw.net/c/180381.html

2.C#Winform使用扩展方法自定义富文本框(RichTextBox)字体颜色

http://www.cnblogs.com/bobositlife/p/csharp-winform-change-richtextbox-font-color-using-static-extension-method.html

3.RichTextBox.SelectionLength 属性

https://msdn.microsoft.com/zh-cn/library/windows/desktop/system.windows.forms.richtextbox.selectionlength(v=vs.100).aspx/html

　　用的时候都不是一帆风顺的，别人的：

　　1.

private void AddTextAndSetFonts(RichTextBox rtx, string addStr, Color textColor)
{
int start = rtx.Text.Length;
int length = addStr.Length;
rtx.AppendText(addStr);
rtx.SelectionStart = start;
rtx.SelectionLength = length;
rtx.SelectionColor = textColor;
}
2.

public static void AppendTextColorful(this RichTextBox rtBox, string text, Color color, bool addNewLine = true)

private void ModifySelectedText()

{

   // Determine if text is selected in the control.

   if (richTextBox1.SelectionLength > 0)

   {

      // Set the color of the selected text in the control.

      richTextBox1.SelectionColor = Color.Red;

      // Set the font of the selected text to bold and underlined.

      richTextBox1.SelectionFont = new Font("Arial",10,FontStyle.Bold | FontStyle.Underline);

      // Protect the selected text from modification.

      richTextBox1.SelectionProtected = true;

   }

}

 尤其要注意代码顺序不然会有问题，还有就是设置后要取消。经过自己改造后的：

private void AppendTextUnderline(RichTextBoxEx rtBox, string text, Color color, bool addNewLine)
{
if (addNewLine)
{
text += Environment.NewLine;
}
// int start = rtBox.Text.Length;
// int text_length = text.Length;

rtBox.SelectionStart = rtBox.Text.Length;
//rtBox.SelectionLength = text_length;
rtBox.SelectionColor = color;
rtBox.SelectionFont = new Font("Arial", 10, FontStyle.Bold | FontStyle.Underline);
rtBox.AppendText(text);
rtBox.SelectionColor = rtBox.ForeColor;
rtBox.SelectionFont = rtBox.Font;//恢复默认字体，否则会影响到后面
}