La ropa de color llena la cesta de ropa.

Because ropa is a collective noun in Spanish. It refers to clothing or clothes as a single mass, even though in English clothes is plural.

So Spanish says:

• la ropa = the clothes / the clothing

That is why the verb is singular too:

• La ropa ... llena
  not La ropa ... llenan

This is very common with ropa:

• La ropa está sucia = The clothes are dirty
• La ropa blanca = white clothes

In this sentence, de color means coloured / colored or non-white clothes.

Literally, it is of color, but in Spanish this is a normal way to talk about laundry categories:

• ropa blanca = white clothes
• ropa de color = coloured clothes

So this is not just any clothes that happen to have a colour. It often refers specifically to the category of clothes you wash separately from whites.

Because the subject is la ropa de color, and ropa is singular.

Here, llena is the third person singular form of the verb llenar:

• yo lleno
• tú llenas
• él/ella llena

So:

• La ropa de color llena la cesta de ropa
  = The coloured clothes fill the laundry basket

Even though English uses clothes as a plural noun, Spanish uses singular ropa, so the verb stays singular.

Here it is a verb: llenar = to fill.

So:

• La ropa de color llena la cesta de ropa
  = The coloured clothes fill the laundry basket

But llena can also be an adjective meaning full when it agrees with a feminine singular noun:

• La cesta está llena = The basket is full

A learner might confuse the two because the form looks the same. In your sentence, it is a verb because it comes between the subject and the object:

• subject: La ropa de color
• verb: llena
• object: la cesta de ropa

Spanish often uses the definite article more broadly than English.

Here:

• La ropa de color = the coloured clothes
• la cesta de ropa = the laundry basket / clothes basket

In Spanish, using la sounds natural because both nouns are being treated as specific, identifiable things in the situation.

Spanish often uses articles where English might sometimes leave them out or choose a different structure.

The two uses of ropa are doing different jobs:

1. La ropa de color = the coloured clothes
2. la cesta de ropa = the clothes basket / laundry basket

So the first ropa is the thing doing the action, and the second ropa is part of the expression naming the type of basket.

In English, repetition may sound a bit heavy, but in Spanish it is perfectly normal.

Literally, it means the basket of clothes.

But more naturally in English, you would usually say:

• the laundry basket
• the clothes basket

Spanish often uses de + noun where English prefers a noun used like an adjective:

• cesta de ropa = laundry basket / clothes basket
• mesa de cocina = kitchen table
• zumo de naranja = orange juice

So de ropa tells you what kind of basket it is.

Sometimes, but they do not always mean exactly the same thing.

• ropa de color is the most natural expression for coloured clothes as a laundry category.
• ropa de colores suggests clothes of various colours.
• ropa coloreada is grammatical, but it is less natural in this everyday laundry context.

So if you are talking about separating whites from coloured washing, ropa de color is the normal choice.

This follows the very common Spanish pattern:

• subject + verb + object

So:

• La ropa de color = subject
• llena = verb
• la cesta de ropa = object

Spanish word order is often flexible, but this is the most neutral and straightforward order.

You could change the order for emphasis, but the original sentence is the most standard one for a learner to understand.

No. Spanish has several words, and usage can vary by region.

In Spain, cesta is very normal for basket. You may also see:

• cesto = basket, bin, hamper
• canasta = basket, but often in other contexts

For laundry, depending on the region or object, people might also say:

• cesto de la ropa
• cesta de la ropa
• cesto para la ropa

So your sentence is correct, but it is not the only possible way to express the idea.

Both can be possible, but they are not exactly the same.

• cesta de ropa = clothes basket / laundry basket as a type of basket
• cesta de la ropa = the basket for the clothes / the clothes’ basket, sounding more specific in a particular context

Using de + noun without the article often names the type or function of something:

• taza de café = coffee cup
• cepillo de dientes = toothbrush
• cesta de ropa = laundry basket

So cesta de ropa sounds very natural when you mean the kind of basket.

Because it comes right after ropa, and together they form a noun phrase:

• la ropa de color

This whole group means the coloured clothes.

Then after that comes the verb:

• llena

So the structure is:

• [La ropa de color] [llena] [la cesta de ropa]

Spanish often places descriptive phrases with de directly after the noun they modify. Here, de color clearly belongs with ropa.