Basically, which of these is correct?
Jeffs graues Messer
Jeffs graue Messer
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If you are talking about one knife, the first phrase
Jeffs graues Messer (one knife)
is correct. The second phrase cannot be interpreted as a singular. However, it is the correct plural form:
Jeffs graue Messer (multiple knives).
In the title you are referring to the strong declension and whether it is used after Jeffs. The answer is yes. Jeffs does not act as an article but could be replaced by another genitive construction:
des Mannes graues Messer
Please be aware that the genitive can be moved to the back; then an article is needed. In summary, the following forms are acceptable German:
(1) Jeffs graues Messer
(2) des Mannes graues Messer
(3) das graue Messer Jeffs
(4) das graue Messer des Mannes
Phrases (1) and (4) are perfect German; (2) sounds old-fashioned; (3) uses a somewhat atypical positioning.