Private Declare Function SetPixel Lib "gdi32" (ByVal hdc As Long, ByVal X As Long, ByVal Y As Long, ByVal crColor As Long) As Long
Dim s As Long, d As Long
Private Sub Form_Load()
d = RGB(255, 255, 0) 'замените переменную d на любой нужный вам цвет
End Sub
Private Sub Picture1_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
s = SetPixel(Picture1.hdc, X / 15, Y / 15, d)
Picture1.Refresh
End Sub

Private Sub Picture1_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
If Button = 1 Then 'цвет пикселя меняется только при нажатой левой клавиши мыши
s = SetPixel(Picture1.hdc, X / 15, Y / 15, d)
Picture1.Refresh
End If
End Sub