Hi, I know this is an old topic but there's not a whole lot on colourants in HP out there so i thought I share what I've learned so far!
I'm fairly new to soap making myself but I've tried a few different things in my first two batches of HP soap and they've all worked fairly well.
I've used tumeric, food colouring, cocoa powder and mica. I think with powder colour you can go for it a bit. I used about 1 1/2 tsp tumeric in about 2lb of soap once it had cooked (mixed with glycerine to make it smooth and liquid) and it only gave it a slight yellow colour so I would say to get the lemony colour I was after I would add maybe 2 1/2 tsp to 2lb and not be worried about staining.
With the mica, again I wanted a stronger colour, so I would probably go for 2 or 2 1/2 tsp with that too.
Cocoa powder I added 2 tsp to 1lb and again the colour wasn't so strong as I would have liked so I would even go as far as 1tbsp per lb on that for a really chocolately colour. I'm going to try this at some stage and test it to see if it makes a brown mess when you use the soap so I will report back then!
With the food colouring I've used about 2 tsp in 1lb and got an nice colour which is just a little much on the strong side to be called a pastel (if that makes sense to anyone!) My green one came out nice and lime coloured (which for once WAS what I wanted!) and the red saved my mica coloured one from being too pastel coloured.
Generally, I think, what you see when you mix in your colour is what you get, it doesn't change much because there's no lye to interfere.
If anyone wants to see I will post pics when I can (my phone which doubles as my digi-cam is on the way out and won't take pics anymore
so I'll have to wait til Christmas!)
Hope that helps a little for anyone who reads it, I think the best way is probably to do some smaller batches to test the strength of your colour. I wonder if you could melt that back into another batch? Anyone?