Subtractive VS Additive

Today I learned about 2 different kinds of color mixing. Being a photoshop artist who has never painted before in his life, when asked what the 3 primary colors we're I was quick to answer Red, Green and Blue (oh yes, the famous RGB). WRONG! ...well partly. The truth of the matter is that RGB are primary colors, but only in the world of Additive color mixing . Additive color mixing is when colors are created through light (e.g. Computer screen, TV screen, stage lights, etc). But then why was i considered WRONG when saying that RGB are the primary colors. It is different here in the real world, in the physical world. For all physical purposes the primary colors are Red, Yellow and Blue - Welcome to the subtractive world. Subtractive color mixing is used for all physical purposes (e.g. Painting, sculpting, object, basically everything you see in the physical world). This strikes me very strong, especially the additive color mixing. For 2 reasons: 1. I am a photoshop artist and it is good to know that all complementary colors are based on the subtractive primaries rather than the additive (even when we are working with additive mixing) and so are all other color schemes and contrasts and 2. I am a theatre actor and to finally understand how light is mixed on a stage using additive mixing is truly fascinating.