You can store an expression longer than 255 in length to a property. But you can retrieve only 256 back.
Reproducing code:

Public oForm
oForm = Createobject('myForm')
oForm.Show()

Define Class myForm As Form
	DataSession = 2
	Height = 400
	Width = 600
	Add Object myGrid As Grid With RecordSource = 'Customer', ;
		Height = 350, Width = 600
	Add Object getDynExpr As CommandButton With Caption = "Din.Expr",Top =360

	Procedure getDynExpr.Click
		Local lcExpr
		lcExpr = Thisform.myGrid.Columns(1).DynamicForeColor
		Messagebox(Transform(Len(m.lcExpr))+Chr(13)+m.lcExpr)
	Endproc

	Procedure Load
		Use (_samples+'data\customer')
	Endproc

	Procedure Init
		Local lcExpr
                lcExpr = '(IIF(LEFT(cust_id,1)="C" and country != "USA"'+
                        ' and maxordamt >= 1000 and title = "Sales Representative"'+;
			' and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1'+;
                        ' and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1'+;
			' and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1'+;
                        ' and 1=1 and 1=1 and 1=1 and 1=1 and 1=1 and 1=1'+;
			' and ATC("c",Company) > 0, RGB(255,0,0), RGB(0,0,0)))'
		This.myGrid.Columns(1).DynamicForeColor = m.lcExpr
	Endproc
Enddefine

Cetin