The cross on Denmark’s flag is white.
Denmark’s flag, called the Dannebrog, has a white Nordic cross on a red field. The cross extends to the edges of the rectangular flag, and its vertical arm is shifted toward the hoist, or flagpole side. This off-centre cross is the defining arrangement of the Nordic or Scandinavian cross design.
The Dannebrog is among the world’s best-known national flags and is traditionally associated with a battlefield appearance in 1219 at Lindanise, near present-day Tallinn. That story is an important Danish national legend, although the historical development of the flag is more complex. A white-on-red cross is documented in connection with Danish kings by the fourteenth century.
The question can cause confusion because Nordic flags use several colour combinations. Sweden has a yellow cross on blue, Norway has a blue cross edged in white on red, and Finland has a blue cross on white. Greenland’s modern flag is different: it uses red and white horizontal fields with a counterchanged disk, reflecting Greenlandic identity rather than reproducing Denmark’s cross.