The git version of Irssi now comes with 256 colour and true colour support.

If you want to use it, make sure your $TERM environment variable is set up properly. You can check it in Irssi with:

/eval echo $TERM

Inside screen (or tmux), it must be screen-256color. Outside of a multiplexor, it depends on your terminal emulator; xterm-256color, rxvt-unicode-256color or gnome-256color are common sensible settings.

Format codes support HTML-like RGB-Hex notation, you can review on Formats wiki page. Sample: %Zabcdef

With a true colour terminal emulator such as st you can even experience full true colour support. To do that you need to ./configure --enable-true-color on Irssi configuration. Then, inside Irssi, you can /toggle colors_ansi_24bit on to turn it on. Do note, tmux does not support it and will display weird stuff.

Updated scripts for 256 nick colours can be found on my web dump too. You still need to manually configure /set neat_colors to the list of colours you actually want to use; take your pick from the screen shot below and input it like this: X42X3AX5EX4NX3HX3CX32

If you can come up with any amazing 256 colour Irssi themes, contributions are welcome.

After you install your Irssi, test it on a fresh config with the /cubes command, should produce this output:

bases
000102030405060708090A0B0C0D0E0F
cubes
101112131415161718191A1B1C1D1E1F1G1H1I1J1K1L1M1N1O1P1Q1R1S1T1U1V1W1X1Y1Z
202122232425262728292A2B2C2D2E2F2G2H2I2J2K2L2M2N2O2P2Q2R2S2T2U2V2W2X2Y2Z
303132333435363738393A3B3C3D3E3F3G3H3I3J3K3L3M3N3O3P3Q3R3S3T3U3V3W3X3Y3Z
404142434445464748494A4B4C4D4E4F4G4H4I4J4K4L4M4N4O4P4Q4R4S4T4U4V4W4X4Y4Z
505152535455565758595A5B5C5D5E5F5G5H5I5J5K5L5M5N5O5P5Q5R5S5T5U5V5W5X5Y5Z
606162636465666768696A6B6C6D6E6F6G6H6I6J6K6L6M6N6O6P6Q6R6S6T6U6V6W6X6Y6Z
grays
7A7B7C7D7E7F7G7H7I7J7K7L7M7N7O7P7Q7R7S7T7U7V7W7X

The internals of 256 colour support are available on the wiki.