From the Super Mario Wiki, the Mario encyclopedia
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Rhythm is a status effect in Paper Mario: Color Splash, inflicted only by yellow paint in the battle with Roy. While Mario is in this state, he is covered in yellow paint, and he cannot use cards with blue backgrounds. Additionally, Huey appears and dances with Mario. Like all other status effects exclusive to said battle, the effect is temporary and only lasts one turn. The music that plays while this effect is active is an arrangement of the Toad Shuffle theme.
If Mario cannot use any of his cards in the Rhythm state, he will dance over to Roy and hit him with a stick. This animation is only used for this status effect. Being blue while not being able to use any cards will not trigger this.