I suspect there may be cameoflage issues with feeders: Birds also don't see colour as we do, from what I understand, so perhaps there's something to do with intensity of colour too. Blue can be quite a 'strong colour' in a winter garden cf. to green. However, Siskins are quite garish, perhaps a red background is needed to counteract the strong contrasts of black and yellow? My fatball and peanut feeders are red mesh,(Blue/great tit/starling/greenfinch) and popular, but the niger seed feeder was bright red plastic and was never successful so dumped. However, the most popular are the clear plastic feeders that have red or green perches (sunflower and wild seed mix - Tits, Greenfinch, Robin, Starling) and the homemade suet/fat feed which I refill coconut shells with (starling, tits). There must be other variables though, other than colour. Tits seem to prefer more cover and don't like feeding in the open also prefering small seed mix - they also tend not to stay long on the feeder in cold weather or when too many other birds are around but fly back and forth (as they do if they are feeding unfledged young in spring) Greenfinch are OK in open and with other birds but will chuck everything out apart from the Sunflower hearts if it's a seed mix. Wind direction could be a factor: They prefer feeding facing to the wind rather than with thier backs to it.
More important to consider I should think is positioning: Sparrowhawk and Cats - this would take priority for most birds I suspect in deciding whether or not to visit a feeder - ie. what's the risk!!!
I NEVER use netting now for fat ball feeds: It's dangerous and have seen several injuries over the years - wire mesh feeders can be bought that will hold fat balls, but the cheapest and easiest way for me has been to make my own mix (porrige oats, bread, sultanas, seed mix, and lard/any fat left over from cooking) and 'pat' it firmly into the coconut shells (which can be scrubbed easily each time before refilling)