GUI texture is very low quality on iPhone

I am trying to achieve a vignetting effect onto my game. I’ve created a 600x600 vignette frame on Photoshop. On Unity, I’ve imported my PSD, added a GUI texture to my scene, and selected the imported file. I’ve sized the texture to fill the entire screen like this:

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It does fill the entire screen regardless of the resolution/aspect ratio. When I export my game for Mac, there is no problem even at lowest render quality. However, when I export and run my game on iPhone, here is how the vignette frame looks:

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It is a screenshot from iPhone (and if helps, the Unity’s built in player-the play button-also displays the same low quality vignette). Notice the banding of the vignetting. It looks terrible, and I know it’s not about the resolution of the bitmap itself, as my image is 600x600 and does look perfect when I export the game to Mac. This is like the image is resized to something like 10x10 and then upscaled back to fill the screen. Why does this happen and how do I fix it?

I’ve figured out the result. In the imported texture’s inspector, the format for the texture was compressed. I’ve changed it to Truecolor and it started rendering much better.