Mishna Berura 160:1

Introduction:
Mishna Berura discusses three levels of objects which can potentially effectuate a shun mareh to passul the water.
1. Ink
2. Smoke / "Other things"
3. Dust and Dirt

1. Ink
We need to know a little bit more of the metzius to understand this comment of Mishna Berura.
"Deyo"(ink) is actually a solid.
For 5,000 years, artists and writers across Asia have used a glossy, dark black, durable carbon-based ink, first recorded in China’s Neolithic, which is known colloquially as Chinese ink. (Chinese ink is also known as India ink and remains extremely popular for contemporary artists and writers.) Chinese ink has been used from China to Korea to India to South-east Asia, on Buddhist and Jainian scrolls as well as in traditional Chinese calligraphy. Conventional Chinese ink, unlike many other inks throughout history, was made to be stored in a solid form, only to be liquefied into ink when needed.*
In Mishnaic times, it seems like kankantom (copperas) was the main ingredient in ink. (Interestingly, kankantom was not black - it was green, but when mixed with other ingredients, it turned black.)

This is what the Mishna Berura is saying here. If deyo - a solid - falls into the water and changes its color, even if the entire deyo-solid hasn't completely dissolved (and thereby also adding substance to the color change), even just with a discoloration (from the initial contact of the deyo with the water) and the deyo-stick is left intact*, the water is passul because of the discoloration (shinui mareh) alone.

2. Smoke / "Other things"
The next "external object" Mishna Berura discusses is smoke and other things, which are less tangible than ink.
The Shulchan Aruch HaRav holds that even this less intrusive object consists a shinui mareh to passul the water.