Have you defined the solder mask color in Board Setup > Physical Stackup? If so, that will not propagate immediately to the 3D viewer. In the 3D viewer, select Preferences > Choose Colors > Get colors from physical stackup. In that menu you can also define the colors directly.
Edit: it sounds like you are changing the color of the core material, not the solder mask, since you only see the edge change?
First:
There is no relation to what colors you set in KiCad, and the color your final PCB PCB will have.
Second:
What you see in Pcbnew is a combination of multiple layers.
Layers such as F.Mask and F.SilkS have very specific meanings in PCB production.
Make sure you understand what they mean and how to use them.
if you would like the KiCad render to be in RED, what you need todo is set the SOLDER MASK colour.
You have set the BOARD BODY colour to red and you can just see this at the mask clearance points. The SOLDER MASK (ie resist) is what gives the PCB the colour.
Very nice PCB. There does appear to be a clearance issue near the bottom where there is a sliver of etching. Are you sure the fabricator can realise this little detail (EDIT and a few near the top honeycomb)
do you want the card to come back with the yellow lines? you will need to request (as part of the fabrication order, not in KiCAD) ENIG to ensure gold-flash otherwise it will come back silver due to HASL being used
Yes thank you, this was the misconception - I had to change the soldier mask color.
I understand that I will have to specify color when ordering the PCB and this is just for visualising.
Well I figured that it will be manufactured as close as possible, it won’t bother me much if it is slightly off. Anyhow this will be my first PCB panel I have created and ordered so result will be a learning experience.
Final version will be gold plated, but for now I will try with HASL.
And possibly also How does solder mask layer work? (Note this focuses on the normal workflow of letting pads define the mask layer. It might however at least partly apply to your usecase as well)
Be especially aware that you really need to check the gerbers not the 3d viewer! There is postprocessing done on the mask layer on gerber export. Most importantly the minimum mask width (minimum web) is applied only on gerber export. (This will mean that some of your thin “green” slivers might vanish)