What is this gold/yellow thing?

[EzHann]

This is a knife that have this yellow or bit golden surface when I flash it with light. What if this is rust or something else?

https://ibb.co/KzKL1xK
https://ibb.co/9q7BYf6

 

[afishhunter]

Looks like a small bit of surface rust, to me.

 

[jeffsenpai]

This is a knife that have this yellow or bit golden surface when I flash it with light. What if this is rust or something else?

https://ibb.co/KzKL1xK
https://ibb.co/9q7BYf6

 

[Bill Siegle]

Looks a lot like the oxidation colors that happen during tempering of the blade. Could also be a light patina forming.

 

[scdub]

That’s a crazy looking blade!
I plan to start a post about this some day, but I’ve had great luck using distilled white vinegar on rust.
I would recommend pouring a little distilled vinegar in a cup and microwaving it until hot, then wet a rag with some pliers and start wiping the blade with the vinegar. (Pliers are just so you don’t burn your fingers with the hot vinegar).
The vinegar will attack the rust and leave a nice blue/grey patina in it’s place. I’ve even just dunked smaller rusted items into a hot vinegar bath which works well too.
Be aware that this may change the entire finish of your blade, but it can be redone/changed if needed...
Below is a knife just after a vinegar treatment:

 

[EzHann]

Is

Looks a lot like the oxidation colors that happen during tempering of the blade. Could also be a light patina forming.

Is that a bad thing to a knife?

 

[USMCPOP]

Is

Is that a bad thing to a knife?

Not necessarily. If it gets to purple/blue or above it'll be softer. Straw to yellow is probably about right.

The colors are caused by a layer of oxidation on the surface. Hotter & longer makes a thicker layer and it refracts light differently, hence the color change.