The Minhag in Belz is to call the Shabbos after Pesach שבת געלע מצות which translates literally as Shabbos Yellow Matzos.
It’s called so because of the Minhag to bake round Challos for Shabbos and to color the Challah with a Yellowish color.
The Friedige Ruv z”l gave the following reasons. After leaving Mitzrayim we need to bring a Karban Todah just like someone who was captured in prison. The Karban Todah was accompanied with Challos of Chometz; that’s why a person can’t bring a Karban Todah on Pesach, so they brought it after Pesach. That’s why we eat round Challos right after Pesach.
Also, the Matzos that the Yidden had when leaving Mitzrayim had a taste of the מן (Manna) that fell later on. The מן was round, that’s why we eat round Challos to commemorate the mann.
The reason that it’s Yellow is because Yellow is in between white and red. White signifies Rachamim and Red signifies Din. At Yetzias Mitzrayim there was Rachamim for the Yidden and Din for Mitzrim.
There is also the old Minhag of making holes in the Challah of this week with a key and it’s a סגולה for Parnasah. It can be understood with the above reason too. The מן shows that Parnasah is from Hashem and the work we do is just a ‘Kli’ to bring in the Beracha from Hashem. When a person realizes that, they merit more Berachah from Hashem. May we all be Zoche!