WPF VB Formatting only last selection in Richtextbox using TextRange or TextSelection

I'm working in wpf and visual basic. When I select text in RichTextBox i use TextRange:
Code:

Code:

---

dim selection1 as New TextRange(Richtextbox.selection.start,Richtextbox.selection.end)

---

Later:
Code:

Code:

---

selection1.ApplyPropertyValue(ForegroundProperty, brushes.Red)

---

When I am marking text many times the color changes also in the previous selections. I want change color only last selection. How to do this?