I bought this pack of 6 gray Koi watercolour brush pen markers on Amazon because I saw a lot of good reviews there. They are quite good for the price. I paid USD $8.99. I also have the box of coloured markers but that will be in a separate review.
They are water-based markers that use dye inks. So they are not lightfast and the colours will definitely fade in the future. As such, they are good for practice but not good if you want permanent work to archive.
One downside is it's difficult to differentiate the cool from warm grays based on the colours of the caps. While the names of the grays are printed on the body, it's not easy to spot them too. That's why I wrote the names on the cap with Sharpie markers. Now it's easier to identify them.
The brush tip is some sort of sponge. Much like those brush tips from Copic markers.
Here are the different grays in this set.
They are waterbased and they blend quite well together and you can easily create a flat wash with no streaking lines. When you compare to Copic markers, when you over lay the markers even by the tiniest bit, you will see the tones get darker. For Koi brush pens, when you overlay, they will spread out and blend together. Of course if you keep adding layers, then you will still see streaking lines.
Some strokes from the brush tip.
This blending example above is done on fine grain cartridge paper. For best results, it's better to use smoother paper. The paper should be quite durable too because the marker is water-based and if you have too many wet layers, the paper fiber will start to come off.
Here are three sketches each coloured with different sets of grays. The one at the top is cool, middle is warm and the last sketch is also warm but from Copic markers.
For the last sketch, you can see the overlay lines from the Copic markers. The sketches coloured by Koi have smoother transition with strokes resulting in a much even wash. Nice.
The Koi watercolour brush pens are fun to use and not too expensive. Just don't use them for archival work.
I noticed a few places selling the set at crazy expensive price so I'm just going to provide the link to Amazon USA where it's the cheapest.