Unfortunately, there is no easy route other than to repaint the entire wall. Paint changes colour over time due to the present environmental conditions. Oil-based paints change rapidly if in the presence of gas heating. The surfaces actually yellow but areas covered by pictures don't yellow so quickly. Painting with latex paint is really the better choice for interior walls. Sunlight changes all paints colour over time. Other factors change the colour especially in kitchens and bathrooms. Also adding another layer of paint will alter the colour slightly even if the wall was just freshly painted. The best and most effective solution is to repaint the entire room. I spent 10 years in the consumer paint business for a paint manufacturer, so my comments are based on industry experience.