If Richtextbox1.text.contains("Tank") Then
  Tank.ForeColor = ForeColor.red
 End If

Private Function FormatText(ByVal TextToFormat As String, ByVal TextFormat As Integer, ByVal TextColor As Color)
    Dim count As New List(Of Integer)()
    For i As Integer = 0 To rText.Text.Length - 1
        If rText.Text.IndexOf(TextToFormat, i) <> -1 Then
            'If the word is found add the index to the list
            count.Add(rText.Text.IndexOf(TextToFormat, i))
        End If
    Next

    Try
        For i As Integer = 0 To count.Count - 1
            rText.[Select](count(i), TextToFormat.Length)
            Select Case TextFormat
                Case 1
                    rText.SelectionFont = New Font(rText.Font.FontFamily, rText.Font.Size, FontStyle.Bold)
                    rText.SelectionColor = TextColor
                Case 2
                    rText.SelectionFont = New Font(rText.Font.FontFamily, rText.Font.Size, FontStyle.Italic)
                    rText.SelectionColor = TextColor
            End Select
            count.RemoveAt(i)
        Next
    Catch
    End Try
    Return Nothing
End Function

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    If rText.Text.Contains("Tank") Then
        FormatText("Tank", 1, Color.Red)
    End If
 End Sub

Public Sub ColorText(box As RichTextBox, color As Color)
    box.[Select](start, 5)
    box.SelectionColor = color
End Sub

ColorizeWord(RichTextBox1, "Tank", True,
             Color.Red, Color.Black,
             New Font(RichTextBox1.Font.FontFamily, RichTextBox1.Font.Size, FontStyle.Italic))

ColorizeWords(RichTextBox1, {"Tank", "[0-9]"},
              IgnoreCase:=False,
              ForeColor:=Color.Red, BackColor:=Nothing, Font:=Nothing)

''' <summary>
''' Find a word on a RichTextBox and colorizes each match.
''' </summary>
''' <param name="RichTextBox">Indicates the RichTextBox.</param>
''' <param name="Word">Indicates the word to colorize.</param>
''' <param name="IgnoreCase">Indicates the ignore case.</param>
''' <param name="ForeColor">Indicates the text color.</param>
''' <param name="BackColor">Indicates the background color.</param>
''' <param name="Font">Indicates the text font.</param>
''' <returns><c>true</c> if matched at least one word, <c>false</c> otherwise.</returns>
Private Function ColorizeWord(ByVal [RichTextBox] As RichTextBox,
                              ByVal Word As String,
                              Optional ByVal IgnoreCase As Boolean = False,
                              Optional ByVal ForeColor As Color = Nothing,
                              Optional ByVal BackColor As Color = Nothing,
                              Optional ByVal [Font] As Font = Nothing) As Boolean

    ' Find all the word matches.
    Dim Matches As System.Text.RegularExpressions.MatchCollection =
        System.Text.RegularExpressions.Regex.Matches([RichTextBox].Text, Word,
                                                     If(IgnoreCase,
                                                        System.Text.RegularExpressions.RegexOptions.IgnoreCase,
                                                        System.Text.RegularExpressions.RegexOptions.None))

    ' If no matches then return.
    If Not Matches.Count <> 0 Then
        Return False
    End If

    ' Set the passed Parameter values.
    If ForeColor.Equals(Nothing) Then ForeColor = [RichTextBox].ForeColor
    If BackColor.Equals(Nothing) Then BackColor = [RichTextBox].BackColor
    If [Font] Is Nothing Then [Font] = [RichTextBox].Font

    ' Store the current caret position to restore it at the end.
    Dim CaretPosition As Integer = [RichTextBox].SelectionStart

    ' Suspend the control layout to work quicklly.
    [RichTextBox].SuspendLayout()

    ' Colorize each match.
    For Each Match As System.Text.RegularExpressions.Match In Matches

        [RichTextBox].Select(Match.Index, Match.Length)
        [RichTextBox].SelectionColor = ForeColor
        [RichTextBox].SelectionBackColor = BackColor
        [RichTextBox].SelectionFont = [Font]

    Next Match

    ' Restore the caret position.
    [RichTextBox].Select(CaretPosition, 0)

    ' Restore the control layout.
    [RichTextBox].ResumeLayout()

    ' Return successfully
    Return True

End Function

''' <summary>
''' Find multiple words on a RichTextBox and colorizes each match.
''' </summary>
''' <param name="RichTextBox">Indicates the RichTextBox.</param>
''' <param name="Words">Indicates the words to colorize.</param>
''' <param name="IgnoreCase">Indicates the ignore case.</param>
''' <param name="ForeColor">Indicates the text color.</param>
''' <param name="BackColor">Indicates the background color.</param>
''' <param name="Font">Indicates the text font.</param>
''' <returns><c>true</c> if matched at least one word, <c>false</c> otherwise.</returns>
Private Function ColorizeWords(ByVal [RichTextBox] As RichTextBox,
                               ByVal Words As String(),
                               Optional ByVal IgnoreCase As Boolean = False,
                               Optional ByVal ForeColor As Color = Nothing,
                               Optional ByVal BackColor As Color = Nothing,
                               Optional ByVal [Font] As Font = Nothing) As Boolean

    Dim Success As Boolean = False

    For Each Word As String In Words
        Success += ColorizeWord([RichTextBox], Word, IgnoreCase, ForeColor, BackColor, [Font])
    Next Word

    Return Success

End Function