Guillermo del Toro recently participated in a Reddit AMA (ask me anything for the uninitiated) and divulged some details regarding what Hellboy 3 could have been.
"Well, you know, we don't have that movie on the horizon, but the idea for it was to have Hellboy finally come to terms with the fact that his destiny, his inevitable destiny, is to become the beast of the Apocalypse, and having him and Liz face the sort of, that part of his nature, and he has to do it, in order to be able to ironically vanquish the foe that he has to face in the 3rd film. He has to become the best of the Apocalypse to be able to defend humanity, but at the same time he becomes a much darker being. It's a very interesting ending to the series, but I don't think it will happen."
When later asked why, the director responded:
"It is a question that I myself ask of the world many times, but we have gone through basically every studio and asked for financing, and they are not interested. I think that the first movie made its budget back, and a little bit of profit, but then it was very very big on video and DVD. The story repeated itself with the second already, it made its money back at the box office, but a small margin of profit in the release of the theatrical print, but was very very big on DVD and video. Sadly now from a business point of view all the studios know is that you don't have that safety net of the DVD and video, so they view the project as dangerous.
Creatively, I would love to make it. Creatively. But it is proven almost impossible to finance. Not from MY side, but from the studio side. If I was a multimillionaire, I would finance it myself, but I spend all my money on rubber monsters."
I highly recommend the entire AMA, del Toro's is one of the best I have read and offers some great insight to color palettes in his films and his creative process.