Two-tone is irrelevant to the offence. It's
Firearms Act 1968, Section 19:
"A person commits an offence if, without lawful authority or reasonable excuse (the proof whereof lies on him) he has with him in a public place [...] an imitation firearm."
Note: not
realistic imitation. Here's an example of
a conviction for simple possession of a bright orange springer pistol which even the arresting officer conceded was a "toy".
Per the Sheriff's comments, the bar for what constitutes a reasonable excuse is set low in
principle. In practice, it's a complete lottery and you may find yourself in front of anti-gun (or poorly advised) judiciary who aren't minded to think that your excuse - of "want to" - is reasonable. You may get away with "boys will be boys", or you may be looking at 6 months / £400 and a criminal record with a big red "FIREARMS" trigger-word in it.
Also bear in mind that the interpretation of "in a public place" is
very broad: “"public place” includes any highway, road within the meaning of the Roads (Scotland) Act 1984 and
any other premises or place to which at the material time the public have or are permitted to have access, whether on payment or otherwise". The chap above was on the steps of the bank, technically on their private property, but considered public due to the general right of access. I've seen arguments made that the direct route from a property boundary or gate to domestic front doors is also public for this purpose, absent signage withdrawing the implied right of access.
tl;dr - don't. Entirely on secure and ideally secluded unambiguously private land only, with the land-owner's permission. If you're travelling somewhere to shoot (on private land) then be prepared to show evidence of that.