It is probably quicker to look for the character than to step through each cell, then determine if it is in a table and if the cell it is in has the required shading.
Code:
Sub ReplaceInTable()
Dim orng As Range
Set orng = ActiveDocument.Range
With orng.Find
Do While .Execute(FindText:="%")
If orng.Information(wdWithInTable) Then
If orng.Cells(1).Shading.BackgroundPatternColor = RGB(63, 123, 196) Then
orng.Text = " CB"
orng.Collapse 0
End If
End If
Loop
End With
lbl_Exit:
Exit Sub
End Sub