you can change a color in a bitmap with image attributes:

Code:
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim img1 As New Bitmap(PictureBox2.Width, PictureBox2.Height)
Dim att As System.Drawing.Imaging.ImageAttributes = New System.Drawing.Imaging.ImageAttributes
Dim map(0) As System.Drawing.Imaging.ColorMap
map(0) = New System.Drawing.Imaging.ColorMap
map(0).OldColor = Color.White
map(0).NewColor = Color.Red
att.SetRemapTable(map)
Dim gr As Graphics = Graphics.FromImage(img1)
gr.DrawImage(PictureBox1.Image, New Point() {Point.Empty, New Point(img1.Width, 0), New Point(0, img1.Height)}, New Rectangle(Point.Empty, img1.Size), GraphicsUnit.Pixel, att)
PictureBox2.Image = img1
End Sub
End Class