If they are regular textboxes (i.e. Insert > Text Box) you could try this
Sub ChangeText(sht As Worksheet)
Dim shap As Shape
For Each shap In sht.Shapes
If shap.Type = msoTextBox Then
If IsNumeric(shap.TextEffect.Text) Then
With shap.TextFrame2.TextRange.Font.Fill.ForeColor
If CDbl(shap.TextEffect.Text) > 0 Then
.RGB = RGB(0, 255, 0)
Else
.RGB = RGB(255, 0, 0)
End If
End With
End If
End If
Next shap
End Sub