"My shirt is orange."
Translation:Minha camisa é laranja.
I guess saying orange colour 'cor de laranja' is an expression from Portugal, since they just say laranja here.
You can use that, but is not common, only "laranja" is better. There is in Brazil "cor de abóbora (pumpkin)", which is orange.
Minha camisa é cor de abóbora.
Minha camisa é laranja.
What's wrong with "Meu blusa é laranja" ? I.e. assuming the speaker is male?
The pronouns agree with the noun they point to. "Minha" points to "(a) blusa", so it has to be feminine too.
In Portuguese (unlike in English), possessive adjectives correspond to the gender of the object, not of the person. Thus "camisa," a feminine noun, would use "minha" regardless of the gender of the speaker or the person to whom it belongs. "Cinto" would always use "meu" because it is masculine. Make sense?