Your major issue is you don't actually tell us what the SCP object does. You've got a footnote that seems to hint at it, and the title tells me what to expect, but you don't explicitly state it, and I don't understand why. Currently, without the footnote, it's just a magical paintbrush - no more, no less.
I would also advise you to stay away from anything that involves compulsion - because we have so many already. Sometimes, what humans choose to do is more unnerving than what humans are forced to do.
So, onto the nitty-gritty.
SCP-3XXX is to be kept in Containment Cell 3XXX
Why does a paintbrush need a whole cell? Why not just a locker?
The Cell will have walls plated with solid iron as to prevent unauthorized use.
How does that prevent unauthorised use?
SCP-3XXX is not to be taken out of the Containment Cell unless authorized testing is to be held.
This goes without saying for almost all SCP objects, and therefore does not need to be included here.
within 100 meters
That is a huge distance. I assume it's a radius - something you need to include - and can you imagine how much that would interfere with day to day life at that Site? What if a researcher is using a pen to take notes, what if there was a staff meeting or seminar? I would reduce that radius significantly.
In testing sessions with SCP-3XXX, two (2) armed guards are to supervise said testing session in an event of misuse.
1. We don't generally do the numbers in brackets thing anymore.
2. Since we don't know what it actually does, I don't know how someone in-universe is meant to know if/how it is being misused.
twenty (20) cm
With numbers, you should write out between one and nine, then use digits from 10 upwards (unless using units such as measurements, in which case you should use digits). So, in this case, it's just '20 cm'.
nor has there been any evidence that SCP-3XXX has been tampered with.
This feels like a line that isn't needed - it should only be noted if SCP-XXXX has unusual markings or something that could indicate 'tampering'.
The material will evaporate, leaving no residue or vapors, and will fill the brush head of SCP-3XXX.
How does pencil lead or graphite 'evaporate'? How does it look when it comes out?
When SCP-3XXX’s brush is filled with a suitable material, individuals nearby feel compelled to use it.
As mentioned before, try to avoid compulsion if at all possible. Also, how nearby is nearby? If it can 'steal' material from 100 meters away, we also need to know how close people can safely get to it.
And then…that's it. The footnote hints at what SCP-XXXX may do, but you don't actually state it. That's a major issue here. Another issue which you may be able to work on better once you've explained what SCP-XXXX actually does/causes is a lack of narrative - there's no real story holding this article together. Currently, it's just a paintbrush that is a slight annoyance due to the fact it steals materials, takes up a whole cell that could be used with a SCP object that actually needs the space, and makes people paint with it. Where's the actual focus of what it does, where's the story you're trying to tell?