Well, before I beat the E4 I really wanted to try out that thing in Vermillion where you speak to the Machoke, check your Coin-Case, and then you see Ho-oh in color.
Well, i tried it with some overworld pokemon (Like the Milktanks in Moo-Moo farm) and got nothing, but I was in the Goldenrod Dept.Store and accidently slipped my finger in the lift and went to the basment instead, where you all know there is....

A Machoke helping move boxes.

So I did it with the machoke and the game froze, emitting a terrible hi-pitched beeping sound. So I reset, and the title sequence was normal. Anyway, I went on my merry way, and this trainer had soemthing for me on the route where the Moo-Moo farm is. I walked two steps out of Olivine, the game froze up and reset.

The water was black and the shellder and Lapras were red and the Magikarp green. Excitedly, I thought I done it, but after the lapras the game froze again and the screen was all glitchy. If you have ever seen the G/S/C MissingNo. then it looked sorta like that. Pressing A only resulted in more glitchy screens. I turned off the game, back on again, and all was well.

So what does this prove? You CAN do the coin-case thing in Johto....Sort of...cause you only get to Lapras, and you never see Ho-oh/Lugia in colour this way. I cannot guarantee it will work, since the reset in Olivine may/may not have been cause by talking to the Machoke. I havent tried to replicate it, not now I can do the one in vermillion

What is the coin-case thing?
i've never heard of it

You go to Vermillion City, and theres a Machoke there, Talk to the machop, and after check your coin case. The game will reset, the colours will scramnble and you see Ho-Oh in colours on the title screen