IIRC, Riot originally intended to give ekko a grey health indicator for how much his ult would heal him, but decided that between that and the shield, his health bar became too busy and scrapped the idea, instead implementing it later with the kench. Also the grey health portion would be there at all times that his ult was off of cooldown, which would take up a big chunk of the room on his health bar (especially on a squishy ekko -- which is how he was intended to be), which leads to visual clutter and lack of clarity, unlike TK, who simply swaps out red/blue for grey without having to compress his existing health bar.