Apenaŭ li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.

Here apenaŭ means something like hardly, barely, or scarcely.

In this pattern, Apenaŭ ... , ... often has the sense of:

• Hardly had ... when ...
• Barely ... and already ...
• As soon as ...

So:

Apenaŭ li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.

can be understood as:

Hardly had he put the dye on the wool when the color changed.

It emphasizes how quickly the second action followed the first.

Because both actions happened in the past:

• metis = put / placed
• ŝanĝiĝis = changed

Esperanto often uses simple tense forms where English might use a more elaborate structure like Hardly had he put... when.... Esperanto does not need a special past perfect here; the sequence is understood from context and from apenaŭ.

So even though English may prefer had put, Esperanto naturally says metis.

In Esperanto, apenaŭ by itself can set up this kind of hardly... when... relationship, especially with a pause or comma:

• Apenaŭ X, Y.

This is a common compact structure. The comma helps show that the second event follows immediately after the first.

If you wanted, you could also use a more explicit structure such as Tuj kiam..., but the sentence as given is perfectly normal and more dramatic or literary in tone.

Because it is the direct object of metis.

• meti = to put / place
• What did he put? la tinkturon

So:

• la tinkturo = the dye
• la tinkturon = the dye, as the thing being put

The -n marks the direct object.

This is because sur can be used in two different ways:

• sur
  • no -n = location, on
• sur
  • -n = direction or movement onto, onto

Here the verb is metis and there is movement toward a surface:

• sur la lano = on the wool
• sur la lanon = onto the wool

Since he is putting the dye onto the wool, lanon takes -n.

The difference is:

• sur la lano = on the wool as a location
• sur la lanon = onto the wool as a destination

Compare:

• La tinkturo estas sur la lano. = The dye is on the wool.
• Li metis la tinkturon sur la lanon. = He put the dye onto the wool.

This is a very common Esperanto pattern with prepositions of place.

Because ŝanĝiĝis means changed in the sense of underwent a change.

• ŝanĝi can be transitive: to change something
• ŝanĝiĝi is intransitive: to change, to become different

So:

• Li ŝanĝis la koloron. = He changed the color.
• La koloro ŝanĝiĝis. = The color changed.

In your sentence, the color itself is the thing undergoing change, so ŝanĝiĝis is the right form.

The suffix -iĝ- often shows that something becomes a certain way or undergoes a change of state.

So:

• ŝanĝi = to change something
• ŝanĝiĝi = to become changed, to change

Then the ending -is makes it past tense:

• ŝanĝiĝis = changed / became changed

This is a very useful Esperanto pattern. For example:

• ruĝa = red
• ruĝiĝi = to become red, blush

Because the sentence is talking about specific things:

• la tinkturon = the particular dye being used
• la lanon = the particular wool involved
• la koloro = the relevant color, probably the color of the wool or of the dyed material

Esperanto uses la for definite reference, much like English the.

In context, these are not random dye, random wool, and random color; they are the specific ones involved in the situation.

It is related, but you should be careful not to assume it always matches English exactly.

In this sentence, tinkturo is best understood from context as something like:

• dye
• coloring substance
• stain/tinting liquid

Since it is being put onto wool and then the color changes, dye is the most natural interpretation here.

So even if the form looks like English tincture, the practical meaning in this sentence is closer to dye.

Yes. Esperanto word order is fairly flexible, although some orders sound more natural than others.

The given sentence is very natural:

Apenaŭ li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.

But you could also rearrange parts for emphasis, for example:

• La tinkturon li apenaŭ metis sur la lanon, kaj la koloro ŝanĝiĝis.
• La koloro ŝanĝiĝis apenaŭ li metis la tinkturon sur la lanon.

Still, the original version is the clearest and most idiomatic for this meaning.

Yes, but the nuance is slightly different.

• Tuj kiam li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.
  = As soon as he put the dye on the wool, the color changed.

This is straightforward and neutral.

• Apenaŭ li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.
  = Hardly had he put the dye on the wool when the color changed.

This sounds a bit more vivid and emphasizes how immediate the change was.

Grammatically, yes: it presents the color as the thing that changed, without directly naming an agent.

That does not necessarily mean there was no cause. The cause is obviously the dyeing action. But the sentence focuses on the result:

• first, he put the dye on the wool
• then, the color changed

So the construction is about what happened, not about who actively changed the color.

A little, yes.

The pattern Apenaŭ ..., ... is completely correct and understandable, but it can feel slightly more formal or literary than a plainer alternative like:

• Tuj kiam li metis la tinkturon sur la lanon, la koloro ŝanĝiĝis.

In ordinary speech, many speakers might choose the simpler version. But the original sentence is a good, natural Esperanto sentence and worth learning.