これでいかがでしょうか。
Sub 特定文字にカラー()
Dim IntRow As Long
Dim IntColumn As Long
Dim UsedRow As Long
Dim UsedClm As Long
Dim r As Range
Dim Target As String
Dim pos As Long
Dim s As String
'データ入力エリアを取得
With ActiveSheet.UsedRange
UsedRow = .Rows.Count
UsedClm = .Columns.Count
End With
Target = "特定文字"
For IntColumn = 1 To UsedClm
For IntRow = 1 To UsedRow
Set r = Cells(IntRow, IntColumn)
s = r.Text
If s <> "" Then
pos = InStr(s, Target)
Do Until pos = 0
r.Characters(Start:=pos, Length:=Len(Target)).Font.ColorIndex = 3
pos = InStr(pos + 1, s, Target)
Loop
End If
Next
Next
End Sub