This is not possible with TextBox Control.
If you are using Excel 2010 or higher, you can use InkEdit Control.
This is an additional control under Tools> Advanced Controls (Excel VBE).

Once you have added it, it will be available in the toolbar as shown below.

UserForm.
, InkEdit Control, MSDN.
Btw, , ( Char) .
Private Sub InkEdit1_Change()
Dim i As Long
If Me.InkEdit1.Text <> "" Then
For i = 1 To Len(Me.InkEdit1.Text)
Me.InkEdit1.SelStart = i - 1 '~~> identifies the start of selection
Me.InkEdit1.SelLength = 1 '~~> identifies the length of selection
Select Case i
Case 1: Me.InkEdit1.SelColor = RGB(255, 0, 0) '~~> colors 1st char Red
Case 2: Me.InkEdit1.SelColor = RGB(0, 255, 0) '~~> colors 2nd char Green
Case 3: Me.InkEdit1.SelColor = RGB(0, 0, 255) '~~> colors 3rd char Blue
Case 4: Me.InkEdit1.SelColor = RGB(255, 255, 0) '~~> colors 4th char Yellow
Case 5: Me.InkEdit1.SelColor = RGB(255, 0, 255) '~~> colors 5th char Indigo
End Select
Me.InkEdit1.SelStart = i '~~> this puts the cursor to end
Next
End If
End Sub
, , 5.
InkEdit RGB , RGB.
, ; , .
-, Excel 2010, , .
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