[Cannot paint onto new pixel layer]

You are a star - thank you so much!!

 

[Cannot paint onto new pixel layer]

Hello.  I am new to Affinity Photo for IPad Pro, and am figuring out how to use it, which is mainly for altering and adding to photographs of my own paintings.  

I have imported a picture as a JPEG and added a new pixel layer, but cannot make the paintbrush work at all.  I gave up in the end, deleted the layer and tried to draw directly onto the original picture, but could.not paint a true colour (with brush opcaity at 100%).  I wanted to paint a bright red onto a dark green and could only get the brush to paint a very dark colour which looks like it may be a mixture of both.  Any help would be appreciated.

Thank you!