The default scene has a sunlight intensity of 5, which means surfaces will appear anywhere up to 5 times as bright as they are in the colour picker when they are facing directly towards the sun. You can reduce this (or reduce the camera exposure), but this can lead to knock-on effects where it's harder to create photorealistic results. For example, with a darker sun or a darker exposure you'll find that the sky is darker, and you can brighten up the atmosphere to compensate but it will affect the lighting in unrealistic ways. You'll also start to create surface colours that are brighter than they should be in terms of real-world measurements, and this can make it difficult to get realistic contrast in your textures. In reality, green grass should be quite dark on the 0..1 scale (usually no more than 0.25) even though it might appear brighter in photographs due to the lighting and exposure.
Having said that, it might still be useful to reduce the sunlight intensity a bit so that you can get results closer to what you expect.