Increasingly in today's Britain, the word racism conjures up two very differing images for white and black people (I use black here in the political sense). When I explain to white friends that certain things that I have had said or done to me are racist, it is more often that not met with incredulity. How can that be racist?

The truth is the most prevalent forms of racism, which many black people experience today, are the insidious kinds. The sideways looks in predominantly white areas suggesting your presence there is beyond the norm. At work, your position of authority undermined by the all-too-easy questioning of your knowledge and experience. And, in the case of Martha-Renee Kolleh, the knowledge that the reason your cafe has such a low footfall is because you are black; the sight of your skin and not the menu explains why potential customers won't be dining with you after all. Her reaction? To create a sign warning the following: "Attention! Everyone be aware, I am a black woman, and always will be. If you are allergic to black people, don't come in!"

I initially balked at Kolleh's move. I feared it played into the hands of those who long to dismiss the very real problems she is attempting to highlight. She's would be seen as just another angry black woman. Yet, if so, her anger is justified. Though I cannot speak for all black people, I know, from workshops with the group Black Feminists, that far too many feel the same desperation and desire to speak back to the insidious racists.

Though born in London, I consider myself a Yorkshire lass. A few weeks ago, a trip to a popular Dales village reminded me of why being in all-white areas is increasingly something I am reluctant to do. I was with a disabled pensioner in his 80s and my 10-month-old. We needed a place to park that would be accessible. Two white, middle-aged male cyclists occupied the available spot. I walked over with a smile and while pointing to the car, a few metres away, asked if it was OK for them to move so that we could use the parking space. They looked at me, then at the car and continued to talk to each other. As I was standing there numbed by their rudeness, a white man in an open-top sports car pulled by and asked them the same question I had seconds earlier. They genially replied "oh, yes". It all happened so quickly that onlookers would have been fooled into thinking that they had simply not heard me. I won't even mention the number of stares I got just walking around the village and, no, it had nothing to do with my attractiveness or indeed lack thereof.

The room for doubt is what makes this subtle form of racism so deeply damaging. It calls into question your understanding of reality and further delegitimises your experiences in the eyes of others and sometimes yourself. Its nebulous nature means that the moment and wounding has, more often than not, come and gone before you are able to articulate your anger and frustration. It is so hard to challenge. If, indeed, you are able to confront the subtle racist, you are told that "you see racism everywhere", as if to say, it isn't. Worse still, you are called aggressive and if you are particularly lucky, told that you, in fact, are the racist.

Kolleh made a necessary and brave choice from a set of limited options. She chose not to remain silent. Not to be labelled as paranoid, or blowing out of proportion what she knows and feels to be true. Not to accept that she is in denial about the quality of her restaurant. Not to be "objective" when the umpteenth person walked in and then swiftly out after catching a glimpse of her black face. She chose a much more difficult, yet rewarding option. She chose to remain and confidentially challenge. And in speaking back with defiance, she has opened a space for the little town of Ossett to engage in a dialogue. People are now discussing the subtle forms of racism some of its inhabitants enact upon others.

Let us not, however, pretend that such racism only exists in sleepy northern towns and villages. Buying a birthday card in an upmarket gift shop, on one of the most fashionable high streets of Hackney, "one of the most racially diverse areas in the UK", I was "unseen" when a cashier attempted to serve a white customer before me. This despite the fact that I was at the counter, with my wallet in my hands. We are either all too visible or conveniently invisible.