Three main options, depending on how much you want to spend and how much effort you want to go to -
Replace with the ones off the Ti (either search scrapyards, but probably not very common, or buy new; either way, it's not a cheap option unless you get lucky second hand and are prepared to fit them yourself)
Get a garage to paint or powder coat them for you (either in-situ, or by getting them to dismantle them first; there's a company I've seen advertised on the internet that will paint them for you if you disasemble and post them off; cost seems to be £100-£300 depending on dismantling etc.)
Paint them yourself (either in-situ or by dismantling yourself).
I decided to just do it myself, on the car without dismantling. All you need is a wire brush, some cleaner, a new brush and a tin of red from Halfords, and some stickers off Ebay - whole job takes about 2-4 hours and costs less than £20. It obviously isn't as good a result as the other two options, but it's bloody cheap, and really effective - you only notice the quality difference if you are purposely looking from closer than a few feet away. Worth doing on any 159 with any wheels IMO.