Ben 837613, you are describing British usage only. Here are some explanations from Americans (who know what the word "trousers" means to Brits but almost never use it, unless they are talking about uniforms or dressing for a formal event):
I don't think we should assume this question is about dressing smartly. Just allow both "pants" and "trousers" as accepted answers (which I hope they now do).
I agree but across the English-speaking world (which is more than just Britain and the States), "pants" is a colloquialism for trousers and if it is to be accepted, should be at a more advanced stage of the course. "Pants" is used in the UK (typically in the north) to mean trousers but it is still colloquial. In all the languages I have learned, being aware of colloquial use is important but you.need the basics first.
Yes, it means 'pants of gray', following the same pattern as 'eyes of blue'.
Apart from the adjectives '-eusi' (black), '-eupe' (white) and '-ekundu' (red), Swahili colour terms are nouns based on real things with that colour. The noun 'kijivu' means ash, so gray is 'rangi ya kijivu' = the colour of ash. To use these nouns as a colour you have to include 'ya' (or za/cha/vya /la/wa, depending on the preceding noun).
Edit: The basic word for ash is 'jivu' (plural 'majivu', ashes).
The word 'kijivu' seems to have evolved from this to mean the color itself (but they still write 'rangi ya kijivu'). It translates as:
having a color somewhere between white and black, as the ash of an ember