The blue ones are metal film resistors, whereas the fawn ones are carbon film. Metal film resistors generally have higher power handling per unit size, hence the confusion.
Most resistors in most amps don't dissipate much power, so you would probably be fine with 1/4 W, especially in a solid state amp which runs at lower voltages than a valve amp.
The main reason I use 1 W resistors in valve amps is the voltage compliance. Most 1/2 resistors are only rated at 350 V, which is less than the B+ in most valve amps. OK, MOST of the resistors won't experience such high voltage under normal use, however the extra cost is small in the scale of things.
Fender use mainly 1/4 resistors (except of plate load resistors which are 1/2 W) in their production amps. We regularly have to replace a blown resistor in the PI of Blues Deville which have a B+ around 520 V. We think that these fail due to the voltage stress as they are dissipating well within their rated capacity.
