Comparing various observed spectral indices (α). The used spectral indices are listed in Table 4. Shown are YSO candidates and false positives, with symbols like in Fig. 3. A slope one line is given (black solid line), highlighting where different spectral indices give the same value. The range for flat-spectrum sources is indicated by dashed lines. Two top rows: comparison of Spitzer and WISE spectral indices, covering approximately the same spectral range. The protostars contain sources with declining α in some spectral ranges. We found that most of these are influenced by visible outflows. The solid orange outline (last row, third column) indicates a transition disk selection (see text for more explanations).