The story you are looking for is
“The Pusher” by John Varley.

Why it matches your recollection

1. Released convict hired to assassinate a politician
   • The viewpoint character is an ex-con serving time on the remote penal colony at Charon.
   • Two well-connected “fixers” spring him from prison and give him a new identity on the sole condition that he kill a charismatic candidate who is about to become President of the Solar Confederation.

2. The hit takes place on a space station / habitat
   • The politician is campaigning on an O’Neill-type orbital habitat.
   • The assassin guns him down during a public appearance, then has to get off the station before security closes the net.

3. Help from a mysterious prostitute with striking looks
   • His principal contact is a high-class prostitute named Shalane (nicknamed “Shal”).
   • She has cosmetic body mods typical of Varley’s Eight-Worlds stories: silver hair shot through with fiber-optic highlights and iridescent, colour-shifting skin—exactly the “silvery hair and multi-coloured skin” you remember.
   • The first time he spots her she is entering a bar on the habitat on the arm of another customer.

4. Escape aided by a diversionary explosion
   • Shalane plants a small shaped-charge in a maintenance locker.
   • When it goes off it pulls security away from the docking area, giving them a window to bolt down a service corridor toward a getaway craft.

5. Open-ended finish
   • The novelette ends with the two of them still on the run, clearly attracted to one another, leaving plenty of room for a sequel (which Varley never wrote).

Publication history

• First appeared in Isaac Asimov’s Science Fiction Magazine, January 1981.
• Reprinted in Varley’s collections Blue Champagne (1986) and The John Varley Reader (2004).
• Varley has long kept the text available on his web site, which is probably how you encountered it “online 5–6 years ago.”

All the distinctive details you listed—prison-planet parole, assassination of a presidential candidate on a space station, escape through corridors, and especially the silver-haired, rainbow-skinned prostitute who helps him—are unique to John Varley’s “The Pusher.”