Hoy solo me pongo un poco de pintalabios.

Because ponerse is very commonly used in Spanish for putting something on yourself.

• poner = to put
• ponerse = to put on oneself / to apply to oneself / to wear

So:

• Pongo un libro en la mesa = I put a book on the table.
• Me pongo pintalabios = I put on lipstick.

The me shows that the action is being done to yourself. In this sentence, the speaker is applying the lipstick to themselves, so me pongo is the natural form.

Hoy means today, and Spanish often places time expressions near the beginning of the sentence.

So Hoy solo me pongo... is a very natural way to say:

Today, I only put on...

You could move it:

• Solo me pongo un poco de pintalabios hoy.

But the original version sounds very normal and slightly more natural if you want to set the scene first: today.

Here solo means only.

So:

• Hoy solo me pongo un poco de pintalabios = Today I only put on a little lipstick.

About the accent:

Traditionally, some people wrote sólo when it meant only, to distinguish it from solo meaning alone. But modern standard Spanish usually writes solo without an accent in both cases, unless there is real ambiguity.

So in modern Spanish, this is perfectly normal:

• solo = only
• solo = alone

Context usually makes the meaning clear.

Because un poco de is the normal structure for saying a little of / a little bit of before a noun.

So you say:

• un poco de agua = a little water
• un poco de pan = a little bread
• un poco de pintalabios = a little lipstick

Without de, it would sound wrong here.

A useful rule:

• un poco de + noun
• un poco + adjective/adverb

For example:

• un poco de pintalabios = a little lipstick
• un poco cansada = a little tired
• un poco tarde = a little late

Here pintalabios is being treated like a substance/product, not as one individual lipstick tube.

Compare:

• Me pongo un poco de pintalabios = I put on a little lipstick.
• Tengo tres pintalabios = I have three lipsticks.

In the sentence, the speaker means a small amount of lipstick, so singular is natural.

Also, after un poco de, Spanish often uses a noun in this kind of general, mass-product sense.

Yes, pintalabios is a very common word in Spain for lipstick.

It is masculine:

• el pintalabios
• un pintalabios

In other Spanish-speaking regions, you may also hear:

• labial
• barra de labios

But for Spanish from Spain, pintalabios is very common and natural.

Because after un poco de, you normally use the noun without an article when you mean it in a general, non-specific way.

So:

• un poco de pintalabios
• un poco de agua
• un poco de azúcar

If you added an article, it would usually make it more specific:

• un poco del pintalabios que me regalaste = a little of the lipstick you gave me

But in the original sentence, the speaker just means a little lipstick, so no article is needed.

Yes, but it means something slightly different.

• Me pongo un poco de pintalabios focuses on the action of applying it.
• Llevo un poco de pintalabios focuses on the result: I’m wearing a little lipstick.

So:

• Hoy solo me pongo un poco de pintalabios = Today I only put on a little lipstick.
• Hoy solo llevo un poco de pintalabios = Today I’m only wearing a little lipstick.

Both are natural, but me pongo emphasizes what the person applies.

Yes. Echarse is also commonly used for applying products to yourself.

So these can all work:

• Me pongo un poco de pintalabios
• Me echo un poco de pintalabios

The difference is small here. Ponerse is very common for things you put on your body, including clothes and makeup. Echarse is also widely used for things like cream, perfume, makeup, sunscreen, etc.

For a learner, me pongo is an excellent and very standard choice.

Because solo usually goes before the part it modifies, and me is an unstressed object/reflexive pronoun that normally stays attached before the conjugated verb.

So the natural order is:

• solo me pongo

not:

• me solo pongo ❌

You can think of it like this:

• solo modifies the whole verbal idea: I only put on...
• me has to stay close to pongo

So the sentence order is very natural Spanish:

• Hoy + solo + me + pongo + un poco de pintalabios

It behaves a little unusually because it is a compound noun, but in practice you will often see:

• el pintalabios
• los pintalabios

So the singular and plural are often the same in form, and the article tells you whether it is singular or plural.

Examples:

• Tengo un pintalabios rojo.
• Tengo dos pintalabios rojos.

That is another reason why, in your sentence, pintalabios does not look different even though learners may expect a more obvious singular/plural ending.