When you use more than one adjective to designate a single color (like "light blue," "dark green," "pale pink" etc.), neither of the adjectives changes according to the noun it modifies. For example:
Il a les yeux bleu clair et les cheveux brun foncé.
"He has light blue eyes and dark brown hair."
In this particular context, using "pairs of" isn't good idiomatic English. "The pairs of pants/trousers are pale green" just sounds wrong. OTOH (On The Other Hand), "I have several pants that are pale green" doesn't sound as natural as "I have several pairs of pants that are pale green". But then "I have some pants that are pale green" is OK, except you can't tell from that whether you're talking about one pair or two+ pairs.
It's clear from the French that we talking about more than one item of clothing, but even if it were singular, I'd have the same thing to say.
Tilo, I gave the same answer and reported it. I don't think the issue should be with the most natural way of saying something in English - "pairs of trousers" is technically correct and emphasizes the fact that "les pantalons" refers to more than one. The exercise is meant to test our comprehension of the French sentence.
Moreover, the notes explicitly say that it's INCORRECT to translate "les pantalons" as "a pair of trousers" precisely because it means more than one pair of trousers. I too entered "the pairs of trousers", specifically to differentiate it from "un pantalon", and it was marked wrong.
The (pair of) pants/trousers = le pantalon
The (pairs of) pants/trousers = les pantalons
The singularity or plurality of garments is always clear in French, whereas in English, if it's not clear from context, then you may specify this information by adding “pair(s) of“. Just take care when translating, because in French you have to be specific.
I have not heard the word slacks in the UK, only in the US, Australia and New Zealand. In that case the word was synonymous with trousers; i.e. a pair-of-pants with a fly front and belt loops. I am guessing that pants is a more informal general term, so one could have track-suit pants but neither track-suit trousers nor tracksuit slacks would be accepted.
Both blue and green can be light or pale, in English as well as in French. To my understanding, pale means less saturated like tending towards white. Light colours are full saturated, but tend to a neighbouring full colour or away from black (where there are dark colours).
For example take yellow. Pale yellow tends towards white. The paler it is the less distance is there to blank white. Light yellow on the other hand can be used to describe a more distance to orange. To speak in computer terms: take this RGB colour as a reference: ffdd00. It is a very light orange or dark yellow. In comparison, ffff00 can be called light yellow.
Note that the blue channel is completely zeroed. When at least one channel is 0, then the colour is not pale. Now let’s increase the blue in order to move the yellow towards white: ffffaa. This is a pale yellow.
Now for another instance take blue: 0000ff. I we add some green, we get a light blue: 00eeff. The red channel is still 0, so the colour is not pale. Now add 75% red: bfeeff. You see no channel is zero, so it is pale, in this case sky blue.
For green, light can mean farther from black. So taken 00c000 (medium green) as a reference, 00ff00 is farther from black 000000 and can therefore be called light green. Whereas pale greens take all 3 channels (get rid of zeroes): c0ffc0.
For anybody wondering what the hexadecimal numbers mean, see https://en.wikipedia.org/wiki/Web_colors