Everything was muted, and the only two colors Kaworu could see were the blue of his own blood still smeared across his uniform shirt (one of a few souvenirs he was allowed to carry into this liminal space) and the red of the Angel core he had just coughed into his hands. Its glow was still visible after he stowed it away in his pocket.
Even in this disembodied state, and even after several trips being carried around thus, Kaworu’s heart still pounded when he saw Shinji Ikari. This time, Kaworu followed his flickering apparition into the music room where he took piano lessons. Shinji-kun plays piano this time. That’s good to know; I need to make sure I learn before I meet him, he thought, and pressed a blue thumbprint of mostly-dried blood into middle C so that he could find it later. Shinji blinked into view again for a few seconds, and his own thumb brushed over it during his warm-up scales, which caused Kaworu’s core to momentarily glow brighter.
Several indigo fingerprints later, when Kaworu was trying to watch Shinji study through the static, something caught Shinji’s attention, and he turned to look right through Kaworu. By this time, Kaworu already knew that his eyes wouldn’t be blue yet (if they were blue in this loop), but looking at this hazy, grey vision of Shinji still made him subconsciously, self-consciously swipe at the bloody corners of his mouth with his wrist, as if to tactfully downplay his abundance of color and reassure Shinji that he wasn’t trying to infringe upon his territory. This was, of course, ridiculous for two reasons. First, Shinji couldn’t see him anyway, and second, Shinji didn’t own blue (but he did own Kaworu’s red heart).
Some time passed, and gradually the intervals between Kaworu’s sightings of Shinji became greater as the glow of his core became dimmer. But Kaworu had managed that time wisely, and he felt quite secure in the constellation of fingerprints he’d left around the area as a map for his future self. This is the building where he attends school. This is where to buy uniforms (Fashion truly is cyclical! he’d thought, looking at all the high-waisted pants on display in the stores.)This is the practice room he uses. This is the sheet music I should study. These are the duets I want to play with him. This is where I told him I loved him last time.
In the last few seconds before his time ran out, Kaworu withdrew his faded red heart with his hands that were now only stained a faded blue, pushed it into his mouth, tipped his head back, and swallowed, and he saw the colors return to the world right before he woke up again.
In his next life, Kaworu wanders the campus of his new transfer school with Mari Makinami, and he comes upon a blue stain on the brick that only he can see, and he remembers.
Basically, in this ficlet, every time Kaworu dies, he coughs up his core and wakes up in this liminal space where he can see where he’ll meet Shinji next. He gets a limited amount of time to explore it and leave messages for his future self before he has to reingest his core to wake up in his next life, or else he gets stuck there.