If you're talking about the color appearing at the ends of bar stock, that has little to do with thickness.
It has to do with the PNS and/or ASTM grade of the steel:

from the pagasa steel website:

Thanks, I thought it deal with thickness. So red is good?

Noong bumili ako ng square bar, may pagkakaiba sa thickness, nang sukatin namin, yung iba't ibang kulay pero parehas lang ang advertised size nila. Mas makapal ang red kesa sa white. Yan rin ang sabi ng tindera.

I suppose when buying steel bars you just have to be specific on the thickness, its usually by millimeters,
sellers don't follow the inches specs, you will just be disappointed. :furious::rant:

Noong bumili ako ng square bar, may pagkakaiba sa thickness, nang sukatin namin, yung iba't ibang kulay pero parehas lang ang advertised size nila. Mas makapal ang red kesa sa white. Yan rin ang sabi ng tindera.

That's not surprising.
A steel mill that makes the effort to follow higher-grade PNS specifications is also likely to stick to correct dimensions.
If all that a mill does is to manufacture lower-grade steel, or worse, IMPORT lower grade steel from China, then you
will more likely see all kinds of anomalies.

To be clear, "red" stock is only painted red on one end. The other end will be a different color.

I suppose when buying steel bars you just have to be specific on the thickness, its usually by millimeters,
sellers don't follow the inches specs, you will just be disappointed. :furious::rant:

Yun pa...
China exports stock in metric AND english dimensions, and everyone from importers all the way down
to your local hardware store can/will take advantage of the difference.

Some measure it in kilograms per meter. I asked some hardware sales clerk (usually women) about the color coding. They said the cheapest ones (yellow) are sub-standard. The most expensive (red or pink - of the same size and thickness) are good quality.

^ganyan din ang pag-iisip ng mga tindahan dito samin.....sabi pa nga e yung white daw pinakamaganda. e kung tutuusin accodg sa chart na ipinakita ni sir horge e white is the lowest grade.