look into Darrel Taylor's 3 channel software PWM code - it's interrupt based. I'm not sure if it's ready to run on a 18F4550. Your main loop would look something like this:

FOR x = 0 to 255 STEP 5
red = x 'REd fade on,
PAUSE delay
NEXT x

FOR x = 0 to 255 STEP 5
green = x 'green fade on,
PAUSE delay
NEXT x

ect...