I took a look at this longplay and what would also make the game a bit more difficult, would be, if the color-order weren't always the same, but were randomly generated by the C64, before each level. Then the player wouldn't be able, to plan entire moves in advance, which would always play out the same way, if you played the game repeatedly. Because then you'd have to adapt your gameplay to the color-order randomly selected by the C64 each time. This could be realised by a click-function in the game-menu, that would offer a choice between "fixed color-order" and "random color-order".
This suggested "random color-order" could be realized only as a selectable function in the game-menue and the "normal color-order", which is already in the game, could stay like it is. Then users could choose, what they want and both sides (advanced players and normal players) could be satisfied. But okay, all theses things are only suggestions, of course.
The "Full Random" mode turned out great, thanks for adding this. I actually meant to mention this, when it was added to the game back then, but I forgot. Today I played the v2.1 of the game for a longer time again and then it occurred to me, that I wanted to write this here. I almost always play the game in "Full Random" mode now. That way, the player can't memorize the whole gameplay and then it doesn't get boring, because it's different, nearly all the time. Nice game, good job. A multiplayer-mode would be the icing on the cake for this already great game now. Perhaps there's a possibility of that someday? But already now, the game is great.