Subtracting and normalizing images

[meusmuse]

I am doing calcium imaging on cultured neurons transfected with GFP. I use the FITC filter to visualize the GFP, however there is a strong nonspecific background fluorescence that correlates to unhealthy cells. Because of this, I use the Texas Red filter to visualize the nonspecific background fluorescence. I want to subtract the Texas Red image from the FITC image, but I need to normalize the pictures first so their brightness and contrasts are the same, and the only difference is the GFP signal in the FITC image. Is there a way to do this in ImageJ? Thank you!