J − K versus J − [12] colour–colour diagrams with all objects selected after cut #2 and brighter than [12] < 9 mag, and with all known and candidate RCB stars represented. On the left side, RCB SED models are overlayed with a colour scale related to the circumstellar shell temperature. The interstellar extinction arrow is also indicated, it shows that many RCB stars were indeed affected by extinction at the time of the 2MASS epochs. This extinction includes circumstellar and interstellar dust. RCB stars observed in a deep decline have their 2MASS J magnitudes recorded with a magnitude limit. On the right side, the Magellanic extreme-AGB stars are represented with orange dots (see Fig. 3). The selection area of cut #3 is delimited with red lines. Four known or candidate RCB stars that were not selected are marked with their names. This plot shows that all selected objects redder than J − K >  3.5 mag will be contaminated by extreme AGB stars. RCB stars will also be even harder to find with such high extinction.