We use to see kitchen sponge with that yellow spongy part which is covered by a thin abrasive green surface but did wonder before why this everyday cleaning tool has this yellow color?

Yes, it is a good question if you did! Before going directly to solve the yellow color mystery, it is good to know that the first artificial sponge was invented by DuPont company in the 1940s and its manufacturing still-secret till 1952. After that artificial sponges rapidly start replacing the natural sponge for most common household uses. 

What is the origin of the current kitchen sponge?

Originally, washing sponges were made out of sea sponges that entered the kitchen because artificial sponges were not invented. After artificial sponges were invented most people went to the artificial one.

What Does Stand Behind The Yellow Color Of Kitchen Sponge?

Artificial sponge made from Polyurethane foam _ one of the most versatile plastic materials _that usually has bright yellow color whereas manufacturers use it because it can deliver the same yellow color that natural sponge has and people used to see while they were using the original sponge for hundreds of years before invented the artificial.

So when artificially made sponges were created they were made in colors people know and trust and this process continues to this day.