Dear London Underground: Why, oh why, 2 black lines?!

From wikipedia:

[The Northern Line] carries more passengers per year than any other Underground line; 206,734,000 passengers per annum. The Northern line has two routes through Central London and two to the north, making it one of the more complex lines on the system.

I may have been here for just over a year, but I made a rookie mistake this morning. After the precious Victoria Line became suspended I got off at Euston and took the wrong branch of the Northern Line, and ended up going 30 minutes out of my way. The infuriating thing is that at Euston, both lines go south! Come on London underground, did you run out of colours?! Why two black Northern Lines?!! You’ve still got the likes of charteuse and magenta to colour in our little map.