The lesson focuses on the importance of color choices in interior design, emphasizing how primary and complementary colors, such as yellow, can enhance the ambiance of various spaces, including living rooms and bedrooms. It discusses practical applications for using different shades and patterns in curtains, walls, and furnishings to create a cohesive and bright atmosphere. The speaker highlights the ability of colors to affect perceptions of space, recommending specific combinations to achieve aesthetic balance and vibrancy.