As redsnail says - don't try using the internet to diagnose colour blindness. The version of the image you are looking at may have been unwittingly altered (e.g. during jpeg compression) and your computer/monitor may not be able to render the colours properly, so your eye sight may be fine but you could still struggle a little to see the image.
Looking at it myself I struggle to see the 5 (I can make it out knowing that it is there, but can't say I'd definitely be able to otherwise) - and I've been tested for colour blindess without issue twice within the last month (once by an optrician, once by occupational health at NATS) - so I wouldn't worry about it.
I'd also add that I've never in my life been presented an image like that one in an Ishihara test. The other images on that page are much more like what you should expect.