Access controls of a from from another

Hello Gurus, i hope someone will be able to help me on this one !! I am using C# and i am quite new to it.

I have 2 forms: one that acts as the main form and another one that is like a secondary form.

On the main form i have a button when clicked will open the second form, on which i have a pictureBox and a TabControl with 2 tabpages. What i am trying to do is from the main form to open the second form and set the backcolour of the PictureBox that is in the second form to blue and set Tabpage2 that is also on the second form active. How can i access the controls that are on the second form from the main form ?

All help are much appreciated..thanks in advance
Angel