If you just want to do some quick coloring, the simplest solution may be to use the end of the RTB content as a Range and apply formatting to it. For example:
TextRange rangeOfText1 = new TextRange(richTextBox.Document.ContentEnd, richTextBox.Document.ContentEnd);
rangeOfText1.Text = "Text1 ";
rangeOfText1.ApplyPropertyValue(TextElement.ForegroundProperty, Brushes.Blue);
rangeOfText1.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);
TextRange rangeOfWord = new TextRange(richTextBox.Document.ContentEnd, richTextBox.Document.ContentEnd);
rangeOfWord.Text = "word ";
rangeOfWord.ApplyPropertyValue(TextElement.ForegroundProperty, Brushes.Red);
rangeOfWord.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Regular);
TextRange rangeOfText2 = new TextRange(richTextBox.Document.ContentEnd, richTextBox.Document.ContentEnd);
rangeOfText2.Text = "Text2 ";
rangeOfText2.ApplyPropertyValue(TextElement.ForegroundProperty, Brushes.Blue);
rangeOfText2.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);
If you are looking for a more advanced solution, I suggest reading this Microsoft Doc about Flow Document, as it gives you a great flexibility in formatting your text.