the silk is more reflective, it does not change with transparency, but i got what you mean. on the tights for modern female have from 10to 100% strongness.
making postwork is more easy, but when i see it would look like nice in transparent, i make originally as transparent (few wings, few accesories, etc).

BuT: if you have any gfx related question, i am gladly answer. If you dont stick to GIMP (what is free), you can switch to Photoshop (the CS2 version is also FREE).