Apex domains cannot have CNAME records.
Question and response
Question
dig colordns.de. NS
Answer
colordns.de. 5 NS ns1.colordns.de.
colordns.de. 5 NS ns3.colordns.net.
colordns.de. 5 NS ns2.colordns.eu.
Authority
The response has no authority records.
Additional
ns1.colordns.de. 5 A 213.9.14.107
ns2.colordns.eu. 5 A 217.79.214.245
ns3.colordns.net. 5 A 217.79.214.253
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0