How to change the property of a control from a flowlayoutpanel?

Well, the easiest way would be to retain an explicit reference to the buttons you're adding. Otherwise you could add a tag to distinguish them (to be robust against i18n issues). E.g. you can set the tag of "button2" to "button2" and then you can use:

foreach(Control ctl in flp.Controls){if("button2".Equals(ctl.Tag)){
        ctl.BackColor=Color.Red;}

}

I am assuming your problem is to find the actual button again and not setting the background color. You could likewise check for the control being a button and its text being "button2" but if the text can change depending on the UI language that's probably not a good idea.

ETA: Totally forgot that you can use the control's Name property for this as well.

If you just want to change the background color of the button in a response to an event from the button you can just use the sender argument of the event handler, though.

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You can try Control.ControlCollection.Find.

flowLayoutPanel1.Controls.Add(newButton(){Text="button 1",Name="btn1"});
Button btn1 = flowLayoutPanel1.Controls.Find("btn1",true).FirstOrDefault()asButton;
btn1.Text="found!";