Don't change the default color settings unless you know what you're doing. You're just messing everything up.
Set RGB color management policies back to "preserve embedded profiles". That's half of your problem.
While you're at it, set the RGB working space back to sRGB IEC61966-2.1. Not Monitor Color. Yes, the profiles are the same in this case, but the principle is important.
Convert everything to sRGB before posting on the web. That's the other half.
Thanks for replying.
I've set it back to sRGB (the plain one not the monitor sRGB option) and all the preserve embedded profiles are chosen. Essentially the default once more.
Now by convert everything to sRGB before posting on the web did you mean the option under Edit which is Convert to Profile? Because I tried this then saved it and it's still dark. If this wasn't what you meant, sorry if I'm being dim, where is the convert part you mentioned?
A lot of people panic when they see Photoshop displaying differently from other software. But that's normal and expected. Here's what happens:
1. Photoshop is color managed and performs a document profile > monitor profile conversion when it displays. So whatever the document profile is, Photoshop will display correctly - unless you have CM policies set to "off" (...!)
2. Other applications, including many web browsers, aren't color managed and will just send the file numbers directly to the display. This distinction is extremely important, because there will be a slight difference. Again, that's normal. Photoshop is right and the other apps are wrong.
3. Without color management, it all hangs on how your monitor displays color. Most monitors are so close to sRGB that any sRGB file will display roughly correctly. This is why sRGB is the de facto web standard: it can live without color management. And that's why you should always convert web material to sRGB. Yes, that's the Edit > Convert to Profile option. You can also use Save For Web to do this if you check the appropriate boxes.