Обработчик пункта меню Color, форма frmmain:
private
void mnuColor_Click(object sender, System.EventArgs e){
blank frm = (blank)this.ActiveMdiChild;
frm.MdiParent = this;
colorDialog1.Color = frm.richTextBox1.SelectionColor;
if (colorDialog1.ShowDialog() == DialogResult.OK)
{
frm.richTextBox1.SelectionColor = colorDialog1.Color;
}
frm.Show();
}