I will be the first to admit here that British military history and uniformology is far from my principal field, but as someone who has at one point painted up a unit of 95th Rifles and is going to do so again relatively soon, the question spoke to me enough that I went and did some digging. Now, from what I can find, there was no official statement anywhere for why the Experimental Corps of Rifles, latterly christened the 95th Regiment of Foot, were issued with green jackets and trousers. However, given the doctrinal inspirations behind the corps, there does seem to be a reasonable set of inferences to be made about their uniform as well.

The English Military Library for February 1801 includes an account of the formation of the Rifle Corps which notes two principal influences behind its formation under Colonel Coote Manningham and Lt. Colonel William Stewart: the continued maintenance of rifle-armed Jäger units in European armies, including the Austrian, Prussian, and Russian (but notably not the French); and the earlier experience of fighting both with and against rifle-armed troops in North America, with American and Hessian skirmishers using rifled arms. In many continental European armies, especially German ones, there was a great deal of precedent for the Jäger to have uniforms of a different, less conspicuous colour than their line counterparts – Austrian Jäger wore grey while the line wore white, while in the Prussian and Hessian armies, blue-wearing line troops were screened by green-wearing Jäger. While, to stress again, the explicit inspiration was not overtly stated in any document I know of, the most reasonable explanation is that the uniform mandated by Manningham was essentially modelled on north German precedents. An explanation along these lines – though suggesting a somewhat different inspiration – is offered by Willoughby Verner in his 1912 History & Campaigns of the Rifle Brigade, which notes that Stewart had been an observer on Suvorov's campaign in Italy and Switzerland during the War of the Second Coalition in 1799, and would thus have seen grey-jacketed Tyrolean Jäger and green-jacketed Russian Jäger (although to be sure, Russian line troops also wore green) in action.

What this then does, however, is push back the question one step to whether or not the German states were thinking of camouflage when they kitted out their riflemen in green, and unfortunately the well dried up for me on that count. It is the sort of explanation that seems plausible from pure instinct and might explain why green was adopted by multiple German states, why Russia didn't switch its Jägers from green to a different colour, and also why Austria went for grey (also a reasonably concealing colour). But as it stands plausibility is all I can offer on this count.

I would, however, note a significant detail in relation to another aspect of the Rifle Corps' equipment, and that was their rifles. As specified in the original regulations for their dress and equipment, rifle barrels were to be browned – that is, lightly oxidised on the surface (this was done by soaking the barrel in brine and allowing the surface to tarnish). Browning of both rifle and musket barrels would, by the later stages of the Napoleonic wars, be relatively commonplace for British light regiments, with the obvious function being that you would be less likely to be given away by a glint of sunlight reflecting off your gun, and it would also simply be less conspicuous against earth and foliage. Given that this minor camouflaging was also done for red-coated light infantry later on, it's probably fair to suggest that concealment was also a consideration behind the green uniforms of the Rifles back in 1800.

We can also look at precedents from within recent British military practice. Light infantry had been especially important in Britain's wars in continental North America in the 18th century, and newly-raised forces would often be expected to wear green (whereas the regular army's light troops generally retained red). Rogers' Rangers in the Seven Years War (aka French and Indian War) were supposed to wear green jackets, while green jackets were also issued to the Queen's Rangers during the American War of Independence. Granted, green was also used for the Royal Regiment of New York, a Loyalist line unit. But what it does suggest is that there was plenty of precedent within then-recent British military history of issuing green uniforms to specialist light infantry units. In this regard the Rifles were arguably not really a departure from the examples set by Ranger units in North American campaigns.

As for the claim that George III liked the colour green, while there is apparently a common story that George was behind St Patrick coming to be associated with green rather than blue, I can find no evidence that he was personally involved in the selection of the Rifles' uniforms. With regards to officers' proclivities in uniforms, it is worth noting that in general, uniforms were pretty uniform by this stage, and it was largely the quality of manufacture and not the basic appearance of a given regiment or battalion's uniforms over which a commanding officer would have influence. Basically only facing colours (that is to say cuffs, collars, and shoulder straps) could vary at the regimental level. In that regard the rather bold green-and-black of the Rifles does reflect a particularly stark departure, but as noted above, there were both European and British precedents for garbing light infantry specialists, and particularly riflemen, in green.

As a further coda, it is interesting that the first campaign of the Rifle Corps would in fact be in Spain, but during the failed Ferrol Expedition in August 1800 rather than the Peninsular War that began in 1808. Now, it is true that dark green is not necessarily the most intuitive colour for a long campaign in Iberia, but the 95th was not necessarily confined to any one theatre long term – and indeed it was only in Spain for a matter of two days in 1800. The Rifles went on to fight in the Spanish Viceroyalty of the Rio Plata in 1806 and went on the Danish expedition of 1807 before heading to Spain, and parts of the newly-raised 3rd Battalion were sent off to North America in 1814 during the closing stages of the War of 1812. While green might not have been the ideal uniform for Iberia, it was presumably deemed a good choice for a force that could theoretically end up anywhere.