The colors were not copied from anywhere, they were picked, by me, with no reference to any existing art, using a careful process designed to maximize distinguishability. To be specific, I decided that the colors should all be clearly separated in CIELAB lightness and hue, and then chose which lightness to match to which hue based on the gamut of then-current iPhones.
The result is a bunch of “bright” colors. But lots of games want bright colors, because they are the most easily distinguishable, and having easily distinguishable colors makes a game easier to play.
As for the appearance of the bricks, I cooked up a dozen or so possibilities in Photoshop, and let the rest of the team decide which to use. Again, created in a few various styles, but not with any reference to any previous game.
Copying assets doesn't just mean verbatim copy-and-paste. It includes recreating other assets such that they are virtually identical to the source material, as the art assets you apparently created were.
Xio's clone went beyond just rules and gameplay--it copied the dimensions, colors, and various art assets. Comparing screenshots of Tetris and Xio's clone, it would have been very difficult for most people, even those familiar with Tetris, to identify which was the original.