Sett Grit - why does it fill with yellow?

Can anyone explain to me, please, why Sett's Grit gauge turns yellow? I can't find this in his abilities. I've seen this only couple of times, and I am not too sure what exactly I've seen. I think it looks like this: 1. When he fills his Grit for the first time, the bar is white. 2. But when it reaches maximum, it starts to fill anew with golden/yellow bar.
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