Brush Mixer Tool

[brook18]

Im relay new to photoshop and I was following a tutorial on youtube to see how to draw with the mixer brush tool. I used the ALT key to select a circle with a gradient of colour to use that to draw with. I clicked on something and now the option to select colour using the ALT key isn't working anymore? It just picks black , even if I clean the brush tool. I have tried to reset the brush tool loads of times but nothings working. I just can't get the ALT option to select a large area of colour to work anymore? Any help? What did I do?

Thank you

[c.pfaffenbichler]

Could you please post a screenshot taken at View > 100% and with the pertinent Panels (Layers, Channels, Options Bar, …) visible?