There is another interesting Portuguese adjective "alaranjado" translated by "orangey". It means "like an orange in shape, taste or colour". Perhaps it would be useful to talk about someone who has just come back from a particularly sunny holiday where there was ample food - "Ele está alaranjado" = he is the shape and colour of an orange. :-)
No. The adjective "Laranja" does not have a plural form: http://lilinguaportuguesa.blogspot.co.uk/2010/04/nem-toda-cor-tem-plural.html.
I the UK, it would be ill-advised to talk about a girl or woman's "pants" as in the UK pants is used almost exclusively to refer to underwear. Trousers or slacks or jeans if appropriate is perfect. Pants refer to the male undergarment, but is occasionally used as a polite way of referring to knickers. Don't talk about the colour of a lady's pants, or you will cause offence.
In English, "pants" is always plural because it's short for "a pair of pants", referring to the individual legs that come together to make the garment. (It's plural as a noun, but as an adjective it can be singular: a pant leg.)
In Portuguese, it varies regionally whether "as calças" is both singular and plural the way it is in English or if "a calça" is the singular and "as calças" is the plural.