I disagree about winter being the only season modelled, though; most layouts seem to me to be stuck in perpetual high summer. Trees and hedgerows are fully leafed, crops are growing in the fields, grass is green, gardens beflowered (and maidens deflowered, presumably), and manual workers shirtsleeved.
I suspect the original point was that winter is the only season that tends to be consciously and deliberately modelled as a precise point in the year. Its much rarer to see an explicitly spring or autumn layout, and summer tends to be indistinct. As you say many layouts seem to be "summer", but in reality greenery, crops, grass, people's activities, and even light conditions can are all very different between May and August. Yet many "high summer" layouts have vivid green grass and trees, which suit May, at the same time as ripe wheat in the fields etc that suit August or September.
I guess its easy to say "summer" and just choose whatever trees or scatter materials etc suit your fancy, whereas making a decision to model winter implies a lot more effort is going to go into it from the start. From a lot of experimentation, I'd say 75% of the static grass on the market is only good for representing the portion of the year from March to early June. You actually need the "dead grass", "straw" etc colours to represent the vast majority of the summer, as well as autumn and winter (unless you're representing a carefully manicured lawn).
I'm looking at installing the low voltage polychromatic LED lighting on strips you can buy off the Bay of Thief, or similar polychromatic LED spot lighting, all of which can be set to differing light levels and colours using remote control. I figure a dark blue on low setting would give a moonlit night effect, a golden yellow, again half setting, for dawn, a blue-white full setting for a sunny mid-day, or half setting for a dull overcast day and an orangey-pink half setting for dusk, plus a vivid green for when the local chemical works goes pop.
I'm also working out if I should lay a strip of led lights at the back of the embankment as an uplighter, with a strip at the top as a downlighter, with a view to recreating light pollution at night, and more subtle sunrise and sunset effects. In theory it ought to work if I paint the back wall a neutral light grey, so long as each strip had it's own infra-red receiver.
This is exactly what I've been thinking about trying. I'm sure I saw the suggestion of having a gap between the main backscene and a final "silhouette" layer of scenery, with up-lighting LEDs between. I can't remember if this was in Iain Rice's Cameo Layouts book, or Paul Bambrick's Backscenes book, maybe?
Using the uplighters to represent the low glow of a dawn or dust could be really effective, IF done well. I think the key would be having LED strips with a genuine RGB control that allowed you to carefully and slowly modulate the colour, rather than the pre-set vivid colours that some of the remotes you see on eBay listings seem to have. Something more like the (ridiculously over-priced) Philips HUE home lighting systems.
Some of the online LED shops seem to have controllers that let you choose a point around a colour wheel, but is that how it actually works, or are they just a series of pre-set colours (how many?). I saw another that had manual dials to control the separate Red, Green, and Blue components of the light, which seems much more promising. But fiddly.
Control via an Arduino could potentially be very cool - this project seems pretty simple and lets you tweak the level of each colour RGB using values between 0 and 256 and the speed of fades timed in seconds. This should make some really great granular, and gradual fading, control over colour. You could use the arduino to accept input by a series of physical buttons to set off transitions to a particular "time" of day, or simply run a gradual series of fading transitions to represent a whole day. https://learn.adafru...trips/overview