Well, two (analytical!) thoughts:
1. In terms of the general population, it seems probable to me that a majority of people in this country have hair that is standard blonde, mousey brown or brunette. Black hair is marginally less common; red hair is invariably less common (because it is less common genetically); strawberry blonde hair is pretty uncommon. This is just to do with the genetic origins of the population as a whole; in different European countries, you would find different proportions in the ascendant - in most Meditteranean nations, for instance, being black haired is the default, and being blonde is pretty rare. This situation tends to cut both ways; it can be the case that, because a colouring is common, you are far more likely as an actor to face heightened competition from others who are very like you in superficial appearance - I believe that's it's generally felt (with what justification, I'm not sure) there is nothing that the (British) industry is more overinundated with than blonde haired, blue eyed, 20 something young women. From this perspective, being, say, a redhead can be of great benefit because it helps you to stand out from the crowd. But this can be a double edged sword - it is also more likely that casting directors will cast 'conventionally' in 75% of situations, and this often means going for 'representational types', unless a character is meant to be extraordinary. In this country, that probably means that most 'everyman/everywoman' parts will go to blonde haired or brunette performers, because having black or red hair is seen as immediately distinctive. I myself have fairly non-committal brown hair (although it shades into ginger and blonde, so I can safely say it's actually multi-coloured :) ), but thanks to genetics, I do possess what they call in the trade an 'interesting' face (i.e. one that looks not remotely British) and my casting works in exactly the same way - I am not thought of as an 'everyman' actor because I don't look like the majority of people who live and work in this country. It might be a different story in a different country.
2. There is (however we sometimes pretend otherwise) a set of specific characteristics that we associate culturally with certain colours, and hair colour and eye colour do come into this, because we judge characteristics in a story partly on the basis of what the character looks like. Blonde hair is a light colour (as is light brown), and also (as I said above) these are common hair colours for Brits to possess. They seem to connote a certain amount of 'innocence', and an 'everyman' quality - and a large number of heroic characters or victims are cast from among those who have hair of this colour. Black hair, or gingery hair, frequently seems to be felt of as more villainous (I am often told I look villainous because of hair and eye colour) and is felt to be more 'foreign' to British eyes. Ironically, if you have strawberry blonde type hair, you can often be cast as ice-cold and merciless type characters as well. Red hair is cliched as being the mark of a 'feisty' and 'fiery' character. It shouldn't be thought that I'm suggesting that any of these characterisations are remotely accurate, but that people do work on the basis of casting according to 'storybook' characteristics (or, if you prefer, on the basis of the 'four humours', Shakespeare fans!) should not be ignored, when you are shifting hair colour. It may quite possibly net you more offers, but they may be offers for different types of characters to those you are used to. This sort of thing works across the board in the industry - men are less likely to change hair colour frequently, but they do go clean-shaven, grow beards, grow hair long, shave themselves bald - it all alters the way the casting is thought of, even though it's exactly the same person performing the part!!
That may have all been a bit unhelpful in terms of the original question, of course.
My feeling on *that* is: do what makes you feel happiest. If you feel the need to reenergise by changing something about yourself, and that this in turn will buck you up and give you more confidence about selling yourself, then it can only be good for your career. In its own terms, I don't think changing hair colour makes a *massive* difference to your saleability - and, as I said above, the truth of the industry is always that something that makes you stand out (such as dyeing your hair screaming red, for instance) *will* invariably gain you attention, but probably not land you many more parts because you are being seen as ultimately *too* specific. There may be infinitely more competition amongst the 'blond haired standard', say, but it will still be probably one of them who will land the part, over and above the individual sporting the 80's fro or the buzzcut. What changing hair colour may do is make a difference to what you are considered good for *representing*, and so it's worth thinking about all that when you undertake it.
