SCP Foundation

Instances of SCP-XXXX should be kept in a secure concrete containment chamber, with lighting along the ceiling and side walls, as well as one glass window for close-up observation.

Since you're not using the term 'standard euclid containment cell', I'd recommend specifying the dimensions of the cell. Also, this is a super nitpick, but I would change 'close-up' to just 'close'. That sounds weird, but I think it just sounds less clunky when read aloud.

During testing hour, 3-4 Class D personnel should enter the containment chamber. When SCP-XXXX is not being observed, Surveillance footage should be temporarily disabled until observation is resumed.

This confuses me a little. Firstly, when exactly is 'testing hour'? That's a strange term I've never heard before. Change '3-4' to '3 to four', just reads better. 'Surveillance' requires no capitalisation. The biggest issue with this piece of text is that it doesn't belong under containment procedures. Place it in a separate Addendum or perhaps just in the description, and it would make more sense.

chamber,as

Space missing before 'as'.

SCP-XXXX is a black figure, approximately 1.4m tall, with an almost human-like appearance.

Whilst humanoid figures CAN still work- this is thoroughly uninteresting. I can't tell you how many boring cookie-cutter black humanoids I've seen on this site. Spice up the appearance a little! Also, 'human-like' is a vague term. What's 'off' about it? Try being more specific, perhaps.

SCP-XXXX can change its appearance, however, into anything that has been within an eight foot radius of it, vivid with color and animate structure.

Wait, so it can turn into anything that is 'vivid with colour'? What defines a colour as 'vivid'? Does it have to surpass a particular hex code? This is a confusing anomaly. Also, I can't even begin to figure out what you mean by 'and animate structure'. Does the structure have to be animate or does it animate the structure? If I'm assuming everything correctly, a better way to phrase this may be: "SCP-XXXX has been observed to alter its appearance to anything within an eight foot radius, assuming it follows one specific criteria: the object in question must be determined vivid in colour. How SCP-XXXX determines this 'vividness' is currently unknown. Structures SCP-XXXX transforms into are known to become animate."

If any personnel happen to witness SCP-XXXX while it is in the form of the person, the subject will experience violent migraines and nausea. Within twelve (12) hours of the viewing, the subject will then experience discomfort in the lower abdomen, later reporting to see things.

Why? Yeah, that's all I have to say. Why does it have this affect? Granted, you don't need to explain EVERY small detail, but what this article lacks is story or background, and I believe exploring this nausea is a perfect way to expand and make the article more interesting.
This is an issue I see in so many new articles- this is just X object doing X thing. No interview logs or experiments- nothing to intrigue a reader or make them curious. Whilst this concept can be pulled off, it would be especially difficult unless you make an interesting origin story.
One more note- This SCP seems as if it would be involved with 'Are We Cool Yet?'. It's a Group of Interest for The Foundation that you can read up on right here.