This was almost the Palestinian flag
Before the watermelon became the fruit synonymous with Palestine, the orange was often seen as the national fruit instead since the coastline of the region was covered in orange orchards, which became extremely popular in Europe, and were known as the "Jaffa oranges" named after Jaffa, a city in Palestine (also this is where Jaffa Cakes got their name from).
Not only were these oranges extremely popular, but they were also a national symbol for Palestine. Infact, in 1929 someone even proposed remaking the flag with an orange on it (which is the flag that you're currently seeing right now).
The current flag of Palestine was not yet specific to Palestine back in the 1920s, instead it was just a general symbol of Arab resistance (especially against colonialism). But at the same time, many Arab countries were starting to develop national identities, and so they would start with the flag as a base and then modify it slightly to make the flag of their nation. In 1929, the Palestinian newspaper Filastin, remarked that two foreign flags (the British and Israeli flags) were flying on their soil, and yet the British banned them from flying their own flag. They recognized the danger of not having a unifying national symbol, and so they invited the Palestinian people to write into the newspaper with their own proposed flag redesign.
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And what would make for a perfect national symbol? The orange. That's where this design comes from. Compare this flag with the Lebanese flag for example, both contain colors that are representative of Arab identity and their resistance against colonialism, but they also both contain distinctive national symbols to set them apart from the rest of the arab world (the orange for Palestine and the cedar for Lebanon).
So now if we look at other flag proposals that were sent into Palestine, you should see that they have similiar symbolism, the four colours representative of Arab indentity (Red, Green, White and Black) but also the color orange, representative of Palestinian identity.
But these redesigns never really stuck around, and though the 4 colors are still broadly associated with Arab identity, this flag has since narrowed and finally has become that national symbol Palestinians have always wanted.
So why isn't the orange a symbol for Palestine anymore? Well to some extent it still is, as a motif used in art, music or literature (Land of the Sad Oranges for example). But it does seem like the orange has generally faded from public consciousness.
That's probably because in 1948, many Palestinian agriculturalists were forcibly evicted from their orchards, and subsequently, Israel adopted the orange as their own national symbol (Just search Israel orange posters).
But let me leave you with one last observation... when Israel first occupied the West Bank and Gaza, they equipped Palestinians with their own identity cards to distinguish them from Israeli citizens, whereas Israelis got blue cards, Palestinians got orange cards (though green Palestinian identity cards still remain the most common).
As for the modern fruit associated with Palestine, the watermelon, it came in use after Israel banned the flying of the Palestinian flag in 1967, and would go on to ban anything even remotely resembling the symbol of the Palestinian flag, even just using the 4 colors (Red, Green, White and Black). Since watermelons typically contains those colors, it's started being used as a resistance symbol, and in the modern day with the internet, it's only exploded in use.
