How to color different words with different colors in a RichTextBox while a user is writing and raise an event when that colored text is clicked

Given the requirements:

1) A User inserts some text in a RichTextBox Control.
2) If the word entered is part of a pre-defined list of words, that word should change color (so, define a relation between a word and a color).
3) When a mouse Click event is generated on a colored word, an event is raised, to notify which word was clicked.

Possible result (to replicate what’s in the visual example):

Define a custom EventHandler with custom EventArgs:

public class WordsEventArgs : EventArgs
{
    private string m_word;
    public WordsEventArgs(string word) { m_word = word; }
    public string Word { get { return m_word; } set { m_word = value; } }
}

public delegate void WordsEventHandler(object sender, WordsEventArgs e);
public event WordsEventHandler WordClicked;

protected void OnWordClicked(WordsEventArgs e) => WordClicked?.Invoke(this, e);

Subscribe to the event:

this.WordClicked += new WordsEventHandler(this.Word_Click);

Simple Class for the list of words:

public class ColoredWord
{
    public string Word { get; set; }
    public Color WordColor { get; set; }
}

public List<ColoredWord> ColoredWords = new List<ColoredWord>();

Fill the list with some words an related color, then bind it to a ListBox, calling the FillColoredWords() method (in other words, handle a collection of objects that relate pieces of text with Color values):

public void FillColoredWords()
{
    ColoredWords.Add(new ColoredWord { Word = "SIMPLE", WordColor = Color.Goldenrod });
    ColoredWords.Add(new ColoredWord { Word = "COLORED", WordColor = Color.Salmon });
    ColoredWords.Add(new ColoredWord { Word = "TEXT", WordColor = Color.DarkCyan });
    this.listBox1.DisplayMember = "Word";
    this.listBox1.DataSource = ColoredWords;
}

In the KeyPress event, evaluate whether the last entered word is part of the list of words to color:

private void richTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    int currentPosition = richTextBox1.SelectionStart;

    if (e.KeyChar == (char)Keys.Space && currentPosition > 0 && richTextBox1.Text.Length > 1) {
        int lastSpacePos = richTextBox1.Text.LastIndexOf((char)Keys.Space, currentPosition - 1);
        lastSpacePos = lastSpacePos > -1 ? lastSpacePos + 1 : 0;

        string lastWord = richTextBox1.Text.Substring(lastSpacePos, currentPosition - (lastSpacePos));
        ColoredWord result = ColoredWords.FirstOrDefault(s => s.Word == lastWord.ToUpper());

        richTextBox1.Select(lastSpacePos, currentPosition - lastSpacePos);
        if (result != null) {
            if (richTextBox1.SelectionColor != result.WordColor) { 
                richTextBox1.SelectionColor = result.WordColor;
            }
        }
        else {
            if (richTextBox1.SelectionColor != richTextBox1.ForeColor) { 
                richTextBox1.SelectionColor = richTextBox1.ForeColor;
            }
        }
        richTextBox1.SelectionStart = currentPosition;
        richTextBox1.SelectionLength = 0;
        richTextBox1.SelectionColor = richTextBox1.ForeColor;
    }
}

In the MouseClick event, verify whether the event is generated on a colored word.
In that case, raise the custom OnWordClicked() event:

private void richTextBox1_MouseClick(object sender, MouseEventArgs e)
{
    if (richTextBox1.SelectionColor.ToArgb() != richTextBox1.ForeColor.ToArgb()) {
        try {
            int wordInit = richTextBox1.Text.LastIndexOf((char)32, richTextBox1.SelectionStart);
            wordInit = wordInit > -1 ? wordInit : 0;
            int wordEnd = richTextBox1.Text.IndexOf((char)32, richTextBox1.SelectionStart);
            string wordClicked = richTextBox1.Text.Substring(wordInit, wordEnd - wordInit) + Environment.NewLine;
            OnWordClicked(new WordsEventArgs(wordClicked));
        }
        catch (Exception) {
            //Handle a fast DoubleClick: RTB is a bit dumb.
            //Handle a word auto-selection that changes the `.SelectionStart` value
        }
    }
}

In the custom event, you can append the clicked word to a TextBox (or do whatever else you want to do with it):

private void Word_Click(object sender, WordsEventArgs e)
{
    textBox1.AppendText(e.Word);
}