Any dye is going to darken it. So, natural is probably going to be the lightest you can get it. You can mix any of the fiebings pro dye with denatured alcohol to thin it. The thinner you make it the lighter it will be. I am new and I haven't dyed anything that didn't come out darker then I wanted it. However I have learned (from here) that the best way to regulate the color is to thin it down and air brush it. You can apply super thin even coats with an airbrush. It's not an investment I'm willing to take on yet. Mainly because I work in the house. No shop environment. I think my next dying experience I will test out a 12 or 16 to 1 ratio. I tried 7 to 1 with some mahogany dye and it was still darker then I liked.
I haven't used the water based dyes yet. So, I have nothing to offer on those.
Just remember to do a test piece. It's much less painful to trash a test piece then your project.
Also, this hobby is not all together scientific. You will find many different answers to the same question here. People develop their own processes and techniques and apply them for years with success. You will do the same.
You will also learn different pieces of leather take dye differently. So, once you figure out what works for this piece, don't expect it to come out exactly the same from the next hide.
Most of all, have fun.