March 15, 1946, CREAM replaced ULTRA as the codeword for top secret cryptanalytic material.
History:

UMBRA was for many years the overall codeword for Top Secret information in the communications intelligence Sensitive Compartmented Information (SCI) compartment, used for that purpose throughout the Five Eyes community.

UMBRA was retired in 1999, along with SPOKE and MORAY. All three were replaced by the term COMINT, and more recently by Special Intelligence (SI).

Unlike the terms COMINT and Special Intelligence, UMBRA and its predecessors were themselves classified, and in the early post-war period, were frequently changed. Later codewords were retained for several years but were still replaced whenever they were publicly compromised.

Although UMBRA was compromised within a few years of its introduction, it was not replaced but remained in service until its retirement in 1999.

Below is the list of the codewords for the Top Secret COMINT compartment and their dates of service:

ULTRA     (194? – Mar 1946)
CREAM    (Mar 15 1946 – Sep 1947)
GLINT      (Sep 1 1947 – Jun 1949)
COPSE      (Jul 1 1949 – Aug 1950)
ACORN     (Aug 1 1950 – Jun 1951)
SUEDE      (Jul 1 1951 – Jun 1952)
CANOE     (Jul 1 1952 – Sep 1953)
FROTH     (Oct 1 1953 – Dec 1954)
EIDER       (Jan 1 1955 – Jun 1959)
DAUNT     (Jul 1 1959 – Dec 1960)
DINAR      (Jan 1 1961 – Sep 1965)
TRINE       (Sep 1 1965 – Dec 1968)
UMBRA     (Dec 1 1968 – Oct 1999)

Sources:
cryptologicfoundation.org
matthewaid.com