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.
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.