Because Xellos's undeath has depended many many many times on knowing to what degree he may approach the line before becoming toast, and also what degree of politeness is necessary, he puts his hands together in thanks and also makes a very, very quiet promise about wet towels.
"Yes: you get to go home when you've walked three colors successfully. But if you do very well, I'll make you something with those tubers you like tomorrow during your dinner break."