I'd say you have to read about instances and objects again. But for a quick help: Make picturebox1 accessible from outside - (a property on Form2 exposing the PictureBox would be best, but just setting the Modifyer property of your PictureBox to public should do it for you)
After that - try it with Olivier Levrey's Code
form2 newform2 = new form2();
newform2.Show();
newform2.picturebox1.BackColor = Color.Green;
Anyway in a clean solution Form2 would handle this itself (during load, construction, show, etc.) But I think this is not real world code? If you still can't get it done I'll create a complete example for you - just let me know..