From the outset the series has had a dark and gritty visual style with many scenes filmed indoors or at night.
Between important scenes there would often been a cutaway to a series of flashed images accompanied with sounds that helped set the tone and in the case of The Kray's themed second season to evolk imagery of the period being referenced.
Season 4 was notable for it's enhanced version of the cut-aways, used to link scenes throughout the shows run, where the sound was loud and jarring and the flashed up stills more visceral and bloody. This appears to be in line with both Buchan's and Chandler's increasing mental issues from working stressful murder cases.
Many outdoor scenes appear to have been deliberatly shot on overcast days, not hard in the UK, so even when a scene is set outside there is an overcast light to the scene. This is most prevelant when there is a body in the scene or memebers of the team are in the scene.
By season 3 many scenes, especially those in Buchan's basement, were filmed with greenish brown sepia tones with short focal lengths to enhance the scenes depiction of Buchan's stress and mental decline.