Как производить закрашивание определенной части рисунка произвольной формы

Public Class Form1
    Dim Water As New Bitmap("Вода.png")
    Dim Cont As New Bitmap("Бак.png")
    Dim Pic As Bitmap
    Dim G As Graphics
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Pic = New Bitmap(Cont)
        G = Graphics.FromImage(Pic)
        TrackBar1.Maximum = Water.Height
        Label1.Parent = PictureBox1
        PictureBox1.BackgroundImage = Cont
    End Sub
    Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll
        Dim Volume As Integer = Water.Height - TrackBar1.Value
        G.Clear(Color.Transparent)
        G.DrawImage(Cont, New Rectangle(0, 0, Cont.Width, Cont.Height))
        G.DrawImage(Water, New Rectangle(0, Volume, Water.Width, Water.Height), New Rectangle(0, Volume, Water.Width, Water.Height), GraphicsUnit.Pixel)
        PictureBox1.BackgroundImage = Pic
        PictureBox1.Refresh()
        Label1.Text = "Бак заполнен на" & vbCrLf & CInt((TrackBar1.Value / Water.Height) * 100) & "%"
    End Sub
End Class