In the universe of the series I guess the CBI (or Patrick) It comes so soon and so desperately that the blood doesn't have that long to dry and become more "brown" color.

Speaking as an artist, the color red is very present in the series, and I guess specifically the blood in the red john crime scenes had to be striking, recognizable and vibrant.

But in the real world I guess the most plausible explanation is that the fake blood they had must have only been this shade XD

I guess you're right 😂 but I rewatched a few episodes where we can see Jane's bedroom and the color has indeed faded away a little, it was a paler shade of red. Thank you for answering!