"A red shirt and a red coat."
Translation:Lèine dhearg agus còta dearg.
This is a handy page for masculine and feminine clothes and colors. https://www.bbc.co.uk/scotland/colinandcumberland/littleblackbook/wordbank/printouts/clothes_and_colour.shtml
Whether or not a word is feminine or masculine is arbitrary. But you can find general rules here.
I think what you are talking about has to do with the article you would use for masculine words starting with b, f, m, p (which would be "am").
Would it be helpful if the gender of a noun is given when it is introduced in the notes and in https://duolingo.fandom.com/wiki/Scottish_Gaelic? Otherwise we are guessing whether lenition should be used or not. Learning through mistakes is not a bad thing but not the most efficient learning style. Just a thought. Thanks to all who put effort into creating the course.