Mose's blessing, Deuteronomy 33,

Rejoice, O Zebulon, on your jorneys,
And Issachar, in your tents.
They invite their kin to the mountain,
Where they offer their sacrifice of success.
For they draw from the riches of the sea
And the hidden riches of the sand

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Zebulon

Midrash Rabbah Bamidbar 2

His gem was a diamond (YAHALOM).
This flag was white
with a ship drawn on it, as it is written: Zebulon will dwell at the sea shore.

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The identification of YAHALOM as diamond is of course doubtful. It is not reasonable that the Jews in the wilderness of Sinai could come by a diamond so large as to fit into the breastplate, and to carve the name of Zebulon on it. However, in the modern Hebrew, YAHALOM means diamond, and this is the translation Chagall could possibly know by heart. The white color of the tribal flag suggests a clear white gem.

A popular legend tells about the special arrangement between Zebulon and Yissachar. The brothers decided that Yissachar would dedicate himself to the studying of the Tora, and that Zebulon would provide for him. This is hinted by the verse

"Rejoice, O Zebulon, on your jorneys,
And Issachar, in your tents."