In the middle picture, the fish at the top in the middle looks like a fella to me, Plus in the bottom picture right top and bottom middle fish look more the colouration of the males.
You're right though, the colour doesnt seem to be there. I wonder, what are you feeding them? I feed cichlid flakes into the tank for my dwarf cichlids but the silvertips get them too. They are enriched with beta-carrotene (carrot goodness) and this has brought their colours out.
If you look at the middle picture again, you can see the two fish in the middle of the image. The top fish has flatter sides, and its sides are more "toned" up- this is a male. The fish underneath is paler, with more filled/less toned sides this is a girlie. As she fills out more, you may notice (just before she is ready to bear eggs) she will get really reflective mirror like colour to her sides. Her sides will almost look like stretched tin-foil.
It might help you to take a look at them from above. The males tend to be slightly longer and are thinner than the ladies.
If you sit and watch them, you may even see the boys fighting amoungst themselves, and the ladies staying clear. This tends to happen until the ladies are ready for love - when the boys start chasing the ladies instead. Usually 2 boys on one.