‘Slow Horses’ Uses Color to Make Its Conspiracies More Thrilling — and Slough House Even Dingier

For a show with its feet planted so firmly in the mundanities of modern London — pubs, offices set above shops, an Episode 4 action sequence this season that is also a thesis statement on how confusing St. Pancras Station can be — “ Slow Horses ” nonetheless has an expansive feeling to it. The Apple TV+ series’ visual style mirrors its protagonist, in a way.

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It may seem there is no limit to Jackson Lamb’s ( Gary Oldman ) coarseness, but a keen mind is ticking away underneath; likewise, the visual of the show appears ordinary, if slightly gritty, but a lot of work goes into shaping and accentuating the picture so that Diana Taverner’s (Kristen Scott Thomas) MI5 headquarters appears all the sleeker, Slough House looks all the bleaker, and the mysterious shadows in which operatives like Frank Harkness ( Hugo Weaving ) operate run all the deeper.

Director of photography Danny Cohen and colorist Adam Glasman have collaborated on more than 30 projects now, including three seasons of “ Slow Horses .” At this point, they can read each other’s minds as easily as Lamb can read River Cartwright (Jack Lowden). But the color work on the series isn’t just about pulling off the tricks that every DP would like to make a TV show look a little more appealing — darkening uninteresting blank spaces within a frame, making the color consistent between shots, and smoothing out any variables in the lighting. It’s about creating a feeling that deepens our sense of immersion in the story.

For “Slow Horses,” that feeling is all about contrasting Lamb’s grime with Taverner’s polish so that Slough House seems like even more of a dump than it is. This starts with a LUT, or lookup table, that applies an idea of the eventual final color to what everyone can see on the monitors on set.

“The feel of the [LUT] tends toward green, particularly in the shadows,” Glasman told IndieWire. “We really created that look thinking about Slough House, because that’s the constant all the way through the seasons. [Then] as a counterpoint, the idea was that MI5 would be blue, steely, very ‘Bourne Identity’… so essentially the look is shifting toward green cyan, particularly in the shadows.”

While the show LUT is applied to everything during production, the color still needs to be tailored to each environment and sequence when cuts of each episode get to Glasman. “The show LUT will take you part of the way there, but then baking in the contrast levels that we want to apply, matching shot-to-shot? That’s generally all over the place — not because there are any technical faults. That’s just the nature of the material for multiple cameras on multiple days in multiple locations,” Glasman said.

Some colorists will try to create a baseline level of consistency and then go in to apply the final look for the color in a given scene, but Glasman works the reverse way. “My process is to do the whole thing at once. So I’ll go through, decide the level of contrast and the fine adjust of the exact look we want for that scene, and then I’ll just match every shot,” Glasman said.

The work of creating a mood that is as invisible and insistent to the human eye as color can be tricky, however you get there. Glasman had a particular sensitivity on “Spook Street” for making sure River’s sojourn in France felt as oppressive as the child spy training camp he finds there, which is a fine needle to thread while still keeping the show’s cards close to the chest, color-wise. The overall look is neutral enough, but if you stand it up against the scenes shot in London… well, Slough House still looks unappealing, but there’s a little more color in it.

While the core look for the series hasn’t changed over the seasons, Glasman has been able to let a little more light into Slough House, particularly in Seasons 3 and 4. “The first season, I spent a lot of time giving Slough House some shape, vignetting and darkening uninteresting flat corners of the screen. But as we’ve gone along, the production designers adjusted for that, so in ‘Dead Lions’ the rooms started being full of boxes. There’s just much more stuff in there,” Glasman said. “So I’ve had to do a lot less shaping in Slough House as the process has gone on.”

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One of the most challenging tricks with the color on “Spook Street” is pulling off a gradual change from day to night, in an environment with large windows. Those mean that Glasman has a lot of light to manipulate. “We can basically bend the tone and hue curves around any way we want to change the overall color, but then on top of those, if we want to select an area of the frame, we can draw something or key it, mainly using hue, saturation, and luminance, to select a specific area of the frame,” Glasman said.

It’s a bit like creating a mask in Photoshop. Just like in consumer-ready photo editing tools, it’s much, much easier to select and manipulate, say, skin tones than it is to change light coming through big bay windows, particularly if they’ve got any kind of curtains or a frame around it. “And the further you push it, the more difficult it is because you get funky edges,” Glasman said.

Selling that time change with the color grading is one of the least noticeable things about the sequence in question, but it’s important to both the overall flow of the episode and the feeling in the room itself. A lot of invisible work happens in the grading process that didn’t necessarily come with the territory even a few years ago.

“Although what we can achieve is much more limited [than VFX], over the years more and more work that would traditionally have been straightforward visual effects is getting pushed toward us: cosmetics, paint-outs. The tools exist so you can do that work in the DI suite,” Glasman said.

Even on a contemporary-set series like “Slow Horses,” there’s enough time-consuming quasi-VFX that happens as part of the color grading that Glasman works with colorist Finlay Reid to do image clean-up, whether it requires compositing or rotoscoping or other visual effects techniques. “That’s industry-wide. All the DI houses are kind of doing that now,” Glasman said.

The tools used for color grading have evolved significantly since Glasman started as a colorist. But his work on “Slow Horses” shows just how much color can push a series into the best version of itself — or the shabbiest version, as the case may be, “You notice there are some pretty strong looks in there,” Glasman said. And yet, “Slough House is pretty consistent all the way through it — a grimy dump.”

Jackson Lamb wouldn’t have it any other way.

“Slow Horses” is available to stream on Apple TV+.