light, bright, clear morning 2 realizations
ACCEPTED Realization 1
type Cognates
language 1 Classical Arabic
language 2 Jewish Aramaic
lexeme 1 أَصْفَر ʔaṣfar-
lexeme 2 ṣəpar, ṣiprā
meaning 1 yellow; pale
direction
meaning 2 morning
reference BK: I 1347 Jastrow: 1298
comment

It seems plausible that the shift has been intermediately realised via the meaning "clear". 

ACCEPTED Realization 2
type Derivation
language Classical Arabic
lexeme 1 أَصْبَحَ ʔiṣbaḥa
lexeme 2 صُبْح ṣubḥ-, ṣibḥ-
meaning 1 to become clear

أَصْبَحَ الحقّ


the truth has been revealed

direction
meaning 2 morning
reference BK: I 1304
comment

The root ṣbḥ relates to the circle of meanings "morning", “to go up in the morning” etc.