Ni konstatis, ke la koloro ŝanĝiĝis.

Ke introduces a subordinate clause and usually corresponds to English that.

So:

• Ni konstatis = We noticed / We observed / We found out
• ke la koloro ŝanĝiĝis = that the color changed / had changed

In Esperanto, ke is the normal way to connect these two parts of the sentence.

In Esperanto, a comma is very commonly used before a subordinate clause introduced by words like ke, ĉar, se, and so on.

So the comma in:

Ni konstatis, ke la koloro ŝanĝiĝis.

helps mark the break between:

• the main clause: Ni konstatis
• the subordinate clause: ke la koloro ŝanĝiĝis

You may sometimes see slight variation in punctuation style, but this comma is standard and very natural.

The ending -is marks the past tense in Esperanto.

So:

• konstatas = notice / are noticing
• konstatis = noticed

Here, Ni konstatis means that the act of noticing happened in the past.

This is a very important distinction.

• ŝanĝis means changed something
• ŝanĝiĝis means underwent a change / changed by itself / became changed

So:

• La koloro ŝanĝis la etoson. = The color changed the atmosphere.
  • Here the color is changing something else.
• La koloro ŝanĝiĝis. = The color changed.
  • Here the color itself underwent the change.

In your sentence, the color is not acting on another object. It is itself the thing that became different, so ŝanĝiĝis is the correct form.

The suffix -iĝ- often means become or come into a state.

So:

• ŝanĝi = to change
• ŝanĝiĝi = to become changed / to change oneself / to undergo change

Then adding the past tense ending -is gives:

• ŝanĝiĝis = changed / became changed

This suffix is very common in Esperanto and is worth learning early, because it often turns an action into a change of state.

Because la koloro is the subject of the subordinate clause, not the direct object.

In Esperanto, the -n ending marks the direct object. But here:

• la koloro is the thing that changed
• it is not something being acted on by another verb in that clause

So:

• la koloro ŝanĝiĝis = the color changed

If you used koloron, that would suggest the color is a direct object, which would not fit this sentence.

La is the definite article, meaning the.

It is used when the speaker and listener are understood to be talking about a specific thing. So la koloro means the color, not just a color or color in general.

In this sentence, it suggests a particular color already known from context, such as the color of an object, a light, a display, or something being observed.

Because both events are presented as past events:

• Ni konstatis = We noticed
• la koloro ŝanĝiĝis = the color changed

This does not necessarily mean they happened at exactly the same moment; it just means both are viewed from a past point of view.

In English, depending on context, you might translate this as:

• We noticed that the color changed.
• We noticed that the color had changed.

Esperanto often uses the simple past where English might choose either simple past or past perfect, and the exact interpretation comes from context.

Normally, no. In a sentence like this, ke is the standard and expected connector.

English sometimes drops that:

• We noticed the color changed

But Esperanto normally keeps ke:

• Ni konstatis, ke la koloro ŝanĝiĝis.

Leaving it out would sound unnatural or unclear.

They are similar, but not always identical.

• rimarki often means to notice
• konstati often means to ascertain, to observe, to establish, or to note as a fact

So Ni konstatis, ke la koloro ŝanĝiĝis can sound a little more factual or observational than a sentence with rimarkis.

In many contexts, though, both could work.

A rough pronunciation guide is:

shahn-JEE-jis

More exactly:

• ŝ sounds like English sh
• ĝ sounds like the j in judge
• every vowel is pronounced clearly
• stress in Esperanto is always on the second-to-last syllable

So ŝan-ĝi-ĝis has the stress on ĝi:

ŝan-ĜI-ĝis

That fixed stress pattern is one of the nice regular features of Esperanto.