This is a good question. Seo can mean both "this is" and "this". Seo buidhe might be likely to be used to indicate yellow itself or present yellow itself to someone - when teaching children their colours you might point to a patch of yellow and say 'this is the colour yellow' - seo buidhe. Here buidhe functions as a noun. Tha seo buidhe might be more likely to be used to describe a yellow object. Here buidhe functions as an adjective. The seo in seo buidhe is more of a "here is" - "ta-dah! here is the colour yellow". The seo in tha seo buidhe is more of a this thing - "this thing is yellow". I don't know what the Duolingo exercise asked you to do - if you're being asked to translate "this is yellow" into Gaelic, I agree that it's confusing - if you're being asked to translate tha seo buidhe into English there is less of an issue.