I have had this happen with contaminated tongs, blix-contaminated tongs then contacting a sheet during development. But thats the only time I have seen this problem personally, safelight fog can also cause it, colour is best done in total darkness although a #13 safelight filter can be used for brief periods, I have used a 15 watt bulb with no problems... also you can a colour 'safetorch' but they shouldn't be left on or placed too close to the paper, they can fog it too. A safelight filter that has become 'unsafe' can be a source of the problem. Also if you are printing at room temperature you are best to use the kodak kit with no starter.