Two-point spectral index distribution of the Effelsberg sample represented in the [NII]-based diagnostic diagram. The color gradient indicates the α_{[ 4.85−10.45 ]} values. Black dots correspond to sources positions in the diagram. Red thick lines are regression curves of the 15% most flat- and inverted-spectrum sources, while black thick lines are regression curves of the steep-spectrum sources. ± 1σ of regression curves are represented by the outer red and black lines.