If the original paint has been in the wall for a while, then it has changed colour since it was painted. That happens due to exposure to light, and other factors. It's often easier to just re-paint the whole wall than to try to match it. Since your new paint is darker, you could try adding a bit of white and see if you can get closer to the wall colour. If you don't happen to have white wall paint, you can use white acrylic craft paint

I have used a filling paste to fill holes and cracks in a wall once sanded down I repainted the parts of the wall with the exact same paint with a paintbrush

however where I have painted is a shade darker than the whole wall even thou its the same paint. I have undercoated it with white and it still looks different. The decorator used a roller is this why it looks different because I am using a brush?