"I have pink pants."
Translation:Ho i pantaloni rosa.
Actually in Spanish is OK to use "rosas" or "rosa" as plural of the adjective "rosa". Example: "las camisas rosas" and "las camisas rosa" (the pink shirts) are both correct. Same happens with all the other colours named from an object.
The exception comes when you are using a shade or a variation of the color. In that case the color should be always in singular. Example: "Las camisas verde esmeralda" (the emerald green shirts) and "los pantalones azul cielo" (the sky blue pants)
Agreement with noun:
Yellow Giallo Gialla Gialli Gialle
Black Nero Nera Neri Nere
Gold Dorato Dorata Dorati Dorate
Red Rosso Rossa Rossi Rosse
White Bianco Bianca Bianchi Bianche
Azure/Light Blue Azzurro Azzurra Azzurri Azzurre
Gray Grigio Grigia Grigi Grige
Same for all forms:
Brown Marrone but plural marroni for masculine nouns, e.g. I tavoli marroni however, no change for feminine plural nouns, e.g., le sedie marrone
Green Verde but plural verdi, e.g. le sedie verdi, I tavoli verdi
"i" is a form of "the", and a "the" needs to be put before a noun, even if it wouldn't be suitable in English to say a "the" in that case, in Italian a "determiner" must always be put to specify whether a noun is definite or indefinite, (basically between saying "the" and "a",) if that makes sense Also i notice you have said "rosa pantaloni" which would also get you marked incorrect as the adjective must always go after the noun, reverse to English. So it should be "i pantaloni rosa"
The short answer is because!
The better answer is two parts. First of all, when followed by a definite article (the), the prepositions a, da, di, in, and su MUST be combined with the article to make a preposizione articolata. That's how you get words like nei, agli, and sullo: they are just in the, to the, on the and the like.
The second part has to do with definite vs.indefinite. In the singular it's easy: the vs. a/an or il/lo/la etc. vs un/uno etc. In the plural it's a little more complicated. You can still say the books/i libri, but you can't say a books/un libri. One of the ways to say 'some' in Italian is simply to use the preposition 'di' with the definite article. That's pretty easy, too, if you're actually translating 'some' item. The real problem is that in English we often don't have an indication of indefiniteness, so it can be difficult to know when to insert it in Italian. One way to think of it is that if you are talking about ALL of some thing, as a general or conceptual category we'd use no indicator at all in English, but would use the definite article in Italian. As an example "I like history books." "Mi piacciono i libri di storia." If instead you are talking about some portion (even an infinite portion, if you can wrap your mind around that) of the category, then we still often use no indicator in English, but you would use the indefinite plural (some = di+art) in Italian. "We're having pasta for dinner this evening." "Stasera mangiamo della pasta." It's good to know that in Italian this indefinite is usually dropped in the negative. "We have spaghetti, but we don't have bowtie pasta." "Abbiamo degli spaghetti, ma non abbiamo farfalle."
Summary: 1) If you're not comfortable with forming preposizioni articolate, strengthen that skill. 2) Almost always (in Italian) use an indicator. a) If it's a specific item OR a general category, use the definite (lo/gli, il/i, l'/gli, la/le, l'/le). b) If it's NEITHER a specific item NOR a general category, but rather some unspecified partial quantity of all that exists in the world, use the indefinite (dello/degli, del/dei, dell'/degli, della/delle, dell'/delle) - unless it's negative, in which case you'll probably drop it. 3) There are no hard and fast rules in language (or in life), but this is a REALLY useful rule of thumb.
Your answer IS correct. Just as the Italians use 'the' much more often than we do, they use 'some' in places where we might not think it necessary or might not think of it at all. The translation given, "Ho i pantaloni rosa' means 'I have THE pink pants' and is incorrect. "Ho dei pantaloni rosa' can mean either 'I have some pink pants' or 'I have pink pants.'
Viola, Blu, and Rosa never change their form, regardless of masc/fem/sing/plural. This is true for "rosa" (pink); but (be careful) not true for rosso/rossa/rossi/rosse (red), which DOES change forms.
Some others (verde) are the same for m/f, but change (in this case to "verdi") for plural, even if it's feminine.
Some colours, such as pink, whose Italian word also doubles as a noun (rosa=pink; (la) rosa = (the) rose) are invariable, so do not agree in gender or number with the subject. Other examples in Italian include viola (purple, violet also flowers viola or pansy), kaki/cachi (khaki), arancione (orange), giallo limone (lemon yellow), cremisi (crimson), marrone (chestnut (brown))