How To Color Spacific Char Charters In Rich Text Box?

Try handling the RTB's MouseDown event as follows:

private void richTextBox1_MouseDown(object sender, MouseEventArgs e)

{

char find = Char.ToLower(richTextBox1.GetCharFromPosition(e.Location));

char find2 = Char.ToUpper(find);

for (int i = 0; i < richTextBox1.Text.Length; i++)

{

if (richTextBox1.Text[i] == find || richTextBox1.Text[i] == find2)

{

richTextBox1.SelectionStart = i;

richTextBox1.SelectionLength = 1;

richTextBox1.SelectionColor = Color.Red;

}

richTextBox1.SelectionStart = richTextBox1.Text.Length;

richTextBox1.SelectionLength = 0;

richTextBox1.SelectionColor = Color.Black;

}