NIR+IRAC colour–colour diagrams of sources in the IRAC sample. The symbols are as follows: disk-less sources are represented by grey dots, sources with anaemic disks by black open circles, sources with thick disks by dark grey crosses, sources with flat spectrum SEDs by filled black stars, and finally, Class I sources by open circles and a cross. The main sequence is plotted as a black line. The diagrams show that sources with circumstellar material can be clearly distinguished from the main sequence population. Sources with anaemic disks populate the disk loci and are not reddened, generating a reddening gap that is particularly striking in panel b). Sources with thick disks also populate the disk loci, but some are also reddened and fan out in the diagram along with flat-spectrum sources and Class I sources.