$e^+e^- \to b\bar b W^+W^-$ events at the Next Linear Collider: colour structure of top signal and irreducible background

We compute complete tree level matrix elements for gg, qLj -+ b6W+W-. We analyze the irreducible backgrounds to top signal at the Tevatron and at the LHC. Their contribution to the total cross section is about 5% at the LHC, due to single resonant channels. Several distributions with contributions from signal and backgrounds are presented.