[Making colours stand out in an image]

Hi, I started to create an image where I made it black and white, then I coloured a certain part of it in.

To do that I used this tutorial here.

 

 

That worked but I have done most of the image now. But I have clicked something and now when I hover over the colour. It changes it back to black and white.

 

I do not know how I did this and when I click undo, it just starts to undo the originally coloured in parts and doesn't change the brush back to what it was

 

Any help will be greatly appreciated.

Thanks