color picker showing the red i want but fills in black

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible? 

»the layers are in RGB«

So the image is RGB? 

@conor_5957 could it be that your 'fill' dialog is working with the background colour and you've set the red you want as the foreground colour, or vice - versa?

I hope this helps neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right' google me "neil barstow colourmanagement" for lots of free articles on colour management Help others by clicking "Correct Answer" if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.

Difficult to say with just that information.  I think a screen shot of your entire workspace and full resolution with layers panel showing might help.