There’s this sentence in one of DuChinese’s readings :
首先，我们要涂粉底液。如果你的黑眼圈很严重，你可以再用一点遮瑕的东西。
It makes me confused why the writer doesn’t add “的东西” when they mentioned “粉底” in the first sentence but added it when they mentioned “遮瑕”... Why didn’t they add “液” behind “遮瑕” just like what they did to “粉底”?
Why not use 东西？Well, in English you also would probably not say, 'apply the foundation creme thingy.' 'apply the foundation creme' suffices. The writer knew it was a liquid,液.
"再用一点遮瑕的东西。" The writer is not sure what exactly this is: creme, powder, liquid, emulsion, so he/she said 'thingy, 东西'
Concealing blemishes is a science. What is best is a quandry:
BLEMISH-HIDING TIP #3: PICK YOUR FLESH-TONED CONCEALER WISELY
When it comes to disguising imperfections such as blemishes on your face, a creamy concealer is always a good idea. A powder can actually give your skin a cakey look and draw more attention to the thing you wanted to hide in the first place. No thanks! After applying your color correcting or highlighting concealer, try the L’Oréal Paris Infallible Total Cover Concealing and Contour Kit, which comes with four flesh-toned shades. Apply the closest match to your skin tone with your makeup blender so that you can cover your pimple from angles that you wouldn’t be able to reach with your finger. And don’t rub the concealer on—be gentle by lightly dabbing and rolling the makeup blender over your skin.
首先，我们要涂粉底液。 First of all, we have to apply the foundation creme.
如果你的黑眼圈很严重，你可以再用一点遮瑕的东西。 If you have very dark rings under your eyes,
你可以再用一点遮瑕的东西。 you can use a little blemish mask to cover them up.
Actually, I think the blemish mask should come first! It's hard to look beeootiiful!
粉底 is a specified object, it is a 'base powder' that you put on your face for cosmetic reason. when it is placed before the noun 液(liquid), 粉底 becomes an 'adjectival noun' that modifies the noun '液'
粉底液 means 'base powder liquid'
东西 (thing) in 遮瑕的东西 is a unspecified object, it can be anything, as long as it can cover the dark eye circles.
遮瑕 (cover flaw) is a verb
遮瑕的 (flaw covering) is an adjective
遮瑕的东西 is a noun phrase ([adj] + [noun]) ; it means "something that covers flaw"
It's a very interesting question.
Do you know 东西南北中 is used for directions, it means east, west, south, north, center. And in ancient China, we used 金木水火土 in corresponding to 东西南北中。
东（金 gold）西（木wood）南（水water）北（火fire）中（土soil）
Only gold and wood can be carried in basket(ancient basket are mostly made from bamboo, many holes), so we used 东西 to represent tangible stuff.
But afterwards, 东西 was used for not only tangible stuff. For example, 他这个老东西，真不是个东西。（that old man, is not a good person.） But when it means non-tangible stuff, it's derogatory.
The short answer is that conventionally "粉底液" refers to a line of cosmetic products that often aim to soften your skin tone. In fact, you can feel free to replace "遮瑕的东西" with “遮瑕霜”, which is a standalone none as well as “粉底液”.
(No, no no no, "粉底的东西" won't work.)
It's not a Chinese question. "东西" here means "anything" that can "遮瑕." The speaker does not restrict it to liquids ("液"). That's all.
Use foundation liquid first if black rim of the eye is serious, use other article again 先使用 粉底液 ，如果黑眼圈严重，再使用其他东西
