There are no colour photographs of Sylvia LD and as you say, you can't trust colourised pictures. They are described as grey in Birkin's book but hazel in Janet Dunbar's biography of JMB. So, neither brown nor blue. However, eye colour can vary depending on the colour the person is wearing, and that is very noticeable in people with hazel eyes. In the absence of real evidence, we'll probably never know!