Richtextbox control modification help

You could do this by selecting each character in the RichTextBox, one-at-a-time, and calling YourRichTextBox.SelectionColor = #SomeColor;

Where you vary #SomeColor using a function to calculate, or look-up, a new color for each character.

However, if you do this, in a RichTextBox with a lot of text, what you will end up with is (very likely) a flickering mess, even if you use some "ancient" technique for reducing RichTexBox flicker like this: [^].

Assuming you have some special purpose in mind, one way you might approach this ... with speed in mind ... is to create a look-up table of characters and colors. That's assuming you always want the same character to be the same color.

You could create a Dictionary like this:

private Dictionary<char, Color> dctCharColor;

Then initialize it somewhere in your code:

dctCharColor = new Dictionary<char, Color>();
{
     { 'a', Color.Green },
     { 'b', Color.White},
     { 'c', Color.Red }
};

And then, for example, you could color the characters with a method like this:

private void ApplyColor(RichTextBox theRichTextBox)
{
    theRichTextBox.SuspendLayout();
    
    for (int i = 0; i < theRichTextBox.TextLength; i++)
    {
        ch = theRichTextBox.Text.ToLower()[i];

        if (dctCharColor.Keys.Contains(ch))
        {
            theRichTextBox.Select(i, 1);
            theRichTextBox.SelectionColor = dctCharColor[ch];
        }
    }
    
    theRichTextBox.ResumeLayout();
}

I would make the choice to set the character colors after the RichTextBox was filled with its content because I'd want to "isolate" file reading and handle any errors in file reading separately.