Replaced original thermostat with an OEM Toyota one. Also did a drain and refill of radiator with Toyota sslc coolant and washed overflow tank with distill water. Went to check coolant level today, found sliver color residue in coolant. Silver color is more obvious when put on paper, marked by green circle in picture below.

What is this residue? Where does it come from? coolant, thermostat, or engine?

The truck is a 04 with has 118k miles, drives and cools normally. Original owner. No leaks. I also did a block test, color looks okay, blue. Regular coolant change every 30k or 3 years.

The coolant change used half gallon of sslc from a jug with recent pink cap (opened a year or so ago), rest is from a sealed sslc jug of older yellow cap packaging from a few years ago. Didn't see silver residue in either jug when pouring.

Any comments and suggestions are appreciated.

Are you the orignal owner? Could be some sort of headgasket sealant?

I am the original owner. No one touched my truck since out of warranty at 36K miles. Didn't find silver residue when draining coolant, which is before I replaced thermostat.

Also did a block test right away, color is blue.

In '04 the V6 had an aluminum block, the V8 had the cast iron. If you have a V6 maybe it's aluminum flakes left over from the manufacture process?

Could be your water pump shaft wobble letting the impeller wear on the housing.

Your coolant shouldn't be orange. Did you use something other than Toyota fluid at some point?