Do hens need a "special" dustbath?

Here's a question from a newbie:

My 3 hens are about 4 months old and have mostly white feathers (they are RIR's crossed with Columbian Rocks), that seem to be getting a bit cream-coloured (touch of yellow). I think this might be oil or grease. Although they seem very clean and don't smell dirty or anything. Do you think it's grease or just the colour of their mature feathers?

I'm not sure if they are getting enough dust bathing because we have a small backyard that is mostly pea gravel pathway and perennial gardens. The flower beds are mostly covered in cedar mulch, except for some dirt areas. The hens used to like to dust bathe in a raised vegetable bed of loose black earth topsoil, but I had to net that off once it was planted. The hens roam our yard when we're outside, and the rest of the time they live in their outdoor run and indoor coop.

Their indoor coop is straw, and their outdoor run is play sand. I had thought that the play sound would be fine for dust bathing, but today I saw them rolling around in the dirt in a corner of the yard, really happy, and it makes me wonder if they don't like the play sand.

Do I need to give them a little "bathing area" filled with some other material? I have a fairly large indoor coop (about 4-feet-by-7-feet) and run (2-feet-by-10-feet) for only three hens, so if necessary I could fit in a baby bathtub or something, and fill it with some other material.

Just wondering if the creamy tinge to their feathers is going to get darker.

I read the post on the 101 board about how you can wash your hens with Dawn, but I seriously don't see these girls putting up with being washed. And to make thing even worse, they are going to be professionally shot for a magazine story on backyard hens so I want them to look their best. Help!

Thanks for any advice you can give.