Wim, you should probably read and study these http://www.zbrushcentral.com/zbc/zb_quicklinks.php?q=-1 but I’m going to give it a go on showing you anyhow.
Try experimenting with a primitive. Ball, cube, whatever, and clicking off and on the different buttons. There is an RGB, M, and MRGB. with RGB turned on, you will be able to paint on the model with color. with M on, you can paint the material onto the object. MRGB is a combination of both. Also make sure ADD or SUB is off, or you’ll be deforming the mesh while painting. Unless you want that. lol.
You can also completely fill the “visible” parts of the mesh by going to the Color menu and clicking “fill object”. Once again, that can be done with the color that you have active in your color palette or the active material in your materials palette. Check which mode you are in: M, RGB, or MRGB. If goof and you want the material to go back to default, go to the flat material and fill it with that. Now if you change materials it will follow once again. Also should mention you can hide other objects (this is if they’re merged) like eyballs, teeth etc,. when you want different materials for different parts. To hide them use Ctrl+Shift+LeftMouseButton. They will need to be made as seperate groups to do this.
Do a search here at ZBC for eyeball materials. There are some great ones, even the non-matcap ones are awesome. For the iris, use the jellybean mat that comes with zbrush or you can make your own similarly. I typically use a combination of matcaps and standard materials and it’s more than sufficient.
Matcaps’ lighting do not change with zbrush lighting system… for matcaps you’re only using the zbrush light(s) to project shadow in your final render. Matcaps emulate lighting within the material itself. It’s typically a picture (sometimes two) of a sphere applied in some purely “magical zbrush way” to the model. When using different materials for each object, it might also be a good idea to check and see if the materials (if they’re matcaps) have the same direction(s) of lighting. Otherwise it may look a little strange. When rendering with matcaps you’re going to want to open your “lights” menu and see which direction it is pointing toward on that material. You may have to move the light cursor once to update it showing the current material. Once you’ve done that, place the provided cursor over where it looks like the light is coming. Now you need to set up your shadows. The last submenu in your light menu (can’t remember now what it’s called) gives you options for your shadows. Raise the “rays” count and lower the “aperature”. Once again, experiment to get the results you want/need. render with “Best Render” and with shadows clicked on in your render menu. I’m somewhat of a noob too, but I’m getting to know my way around this program pretty well. Hope it helped. PS: Read that other link I put at the top. There’s alot of good info there.
Chad