Nvm. Awkward, annoyting solution but I guess I found it.
Simply turn on Draw>Mrgb before you do the fill. ZBrush needs that M in the equation to know that you want to fill with Material. 
Oh no Aurick that wasn’t it, I know you have to turn it on. I’m having this problems with some custom materials. For example I load the material and apply it then it works. But I can’t load another material when I have that one selected cause it will replace it, I don’t even click on fill object but it still replaces it. I have to select one of the materials that comes with zbrush and then load the other custom material I want for a different subtool and apply it. But it works so oh well 
. Ty though.
Wait a minute, this makes no sense to me… you have your subtool selected, ‘M’ selected in the top toolbar and a custom material in your material slot. Fill Object works, but doesn’t stay once the material is changed? Do you accidentally have the object masked and ‘view mask’ turned off or something?
Yep that’s exactly it. If I have the custom material selected, and select another custom material from my folder, it replaces it whiout me clicking on fill object. Tried it many times. I have to select a pre-made material then go and select the other custom one. It might not always happen, but Im 100% sure this is the case sometimes cause nothing else is going on I checked everything. I’m not too worried about it anymore seeing as the solution is so simple though.
The reason the material is changing when you select a new one is that the object hasn’t been filled with a material. Until you do Color>Fill Object with Mrgb active, the materials will change any time you select a new one. It’s just like working with color – the object will change until you’ve filled it. After filling it with a material, you can select other materials without the model changing and then paint these other materials onto specific parts of the model.
If you load a custom material into a slot that holds a material already in use on your model, then the new material will replace the existing one on the model regardless of whether you’ve filled the object or not.
I am having a problem that relates to this. My material changes everything as if nothing was filled when the subtool. I have selected has a texture applied to it. When I turn off the texture, it works as its supposed to. This problem is driving me crazy, as I need to see my image plane in flat color, and the rest of my model in red wax. I am using 3.12 mac. Thanks.
Aurick. No. I do use fill object with MRGB on or just M. It’s the second thing you said. It replaces the one I had if I load it from a folder. But it’s cool, solution is easy.
I’m having the same problem with the all the ImagePlane tools. I turned on Mrgb put the material on Flat Color selected the image I wanted to use in the BrushTxtr then hit FillObject and it changes but the second I change the selected material, the ImagePlane material changes to that material (Im not hitting FillObject the second time when selecting a material).
I have tried hitting fill object on different Sdiv levels for the ImagePlane , saving the scene with it on the right material,and I have tried making a new ImagePlane but it still switches. I have also tried making a new scene with a demo head and tried the same thing but it changes there too. Any ideas? Its not hard to get it to be the right material but it is really annoying to have to keep changing it back. No other subtools are having this problem. Any help would be much appreciated.
I had this too. Turns out I had a texture applied. As soon as I switched it off (tools> texture pallet) everything went back to normal
Hi,
I’m bumping this because I have a similar problem.
When I’m trying to fill (color and/or material) a subtool nothing happen.
Yes I have Mrgb or Rgb checked in (tried both), and no texture is applied.
It doesn’t work on my currents subtools but it works fine on demo soldier 
Am I missing something?
Hi,
I gotta bump this too I’m afraid - I’m having the same problem and cannot find a solution. With some subtools it’ll work fine: I select a material and color, press ‘Mrgb’ and click color>Fillobject, and it works. But with other subtools I do the same thing and they don’t retain the color OR the material (and the little thumbnail in the subtools pallette remains on RedWax matcap regardless of the material chosen). With some subtools when i click ‘fillobject’ they turn black!, even though neither the main or second color was set to black!
I can only think this must be something with certain subtool models? I can’t understand why it works for some subtools and not others when there’s no obvious difference between them. If anyone can offer any help or advice for this it would be soo much appreciated as I’m really stuck for rendering! thanks
Yeah, I have this problem routinely. As for the problem with objects not maintaining their material assignment you can try filling it multiple times with the constant material first and then with the material it is supposed to have. If that doesn’t work my only solution is to export the object out of Zbrush as an obj and bring it back in. That usually does the trick.
As for objects going black when filling despite neither Foreground or Background Color being black - same workflow as above but usually it is quicker to just paint the entire object with a brush.
getting this too. hmmm
*edit - Seems I just have never had a reason to fill with the flat material until now, which the docs says actually erases the material information. Good to know. Other materials fill fine, so no prob here.
Problem came back. Had the file saved as a zproject. Saved out ztool, restarted and opened it with no more problem. Not sure what the deal was.
Same problem here.
Well, I’m on the same ship guys. But as Znub said, switching to a standard material before choose another custom one makes the trick.