Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

uses
   RichEdit;

 procedure RE_SetSelBgColor(RichEdit: TRichEdit; AColor: TColor);
 var
   Format: CHARFORMAT2;
 begin
   FillChar(Format, SizeOf(Format), 0);
   with Format do
   begin
     cbSize := SizeOf(Format);
     dwMask := CFM_BACKCOLOR;
     crBackColor := AColor;
     Richedit.Perform(EM_SETCHARFORMAT, SCF_SELECTION, Longint(@Format));
   end;
 end;

 // Example: Set clYellow background color for the selected text. 
procedure TForm1.Button1Click(Sender: TObject);
 begin
   RE_SetSelBgColor(RichEdit1, clYellow);
 end;