Disclaimer: Not an actual historian, I just like history. I only know the answer for the Iceland.

Iceland didn't have an official flag, but there was a general consensus in Iceland that the colors blue and white were definitely going to be involved.

There were many different ideas for the final design, but an article written by Einar Benediktsson in his newspaper "Dagskrá" in 1897 served to mostly shut down previous form ideas, many of whom included a falcon, which was a popular idea at the time. The falcon itself having been considered an improvement over a cod-based flag used by Jørgen Jørgensen, who briefly claimed Iceland independent in the early 19th century.

Einar pointed out the difference between a flag and coat of arms, which sometimes had a significant overlap in public discussion, and considered the falcon based designs to be not up to standards of civilized christian nations in Europe. He liked the simplicity of a cross-based design, considering it "the most common and practical" form of a flag.

Einar's suggestion was a white cross on a blue background. A flag known locally as hvítbláinn (the white-blue). I can't find any sources after 1897 seriously discussing an alternative form of flag. The final design would be debated for years until it was made official in 1915. At least of the proposed designs included a danish flag in the top left corner, similar to how many former british colonies include the union jack.

The final design was proposed by Matthías Þórðarson in 1906, even if it wouldn't be adopted officially until years later. The colors represent the fire, ice and mountains blue.

Sources (all in Icelandic):
http://www.visindavefur.is/svar.php?id=1454
https://www.forsaetisraduneyti.is/upplysingar/faninn/Saga_fana/
https://is.wikipedia.org/wiki/Hv%C3%ADtbl%C3%A1inn
https://is.wikipedia.org/wiki/%C3%9Eorskaf%C3%A1ninn