OK, first off it IS possible to work out scales of sci-fi ships because within the context of the film/TV/comic/novel universe they exist in they still have to be flown/driven/operated by people. Unless demonstrated otherwise, people are assumed to be the same size elsewhere in the galaxy as those of us here on Earth. Since we can see the pilots in the snowspeeder (along with in an X-Wing, TIE Fighter or any other starfighter with cockpit windows) we can work out a scale for the vehicle from that.
In cases where you can't directly see figures inside the ships - The
Enterprise or a Star Destroyer, for example - scale can be determined by
examining the size of the windows, hatches and other areas of the ship
that people interact with. Again, a boarding hatch has to be "so" big
for a person to fit through it so once that dimension is known (within a
certain margin for error) you can do the math to figure out the
theoretical size of the ship and thereby determine the scale of the
model. This leads to a whole secondary debate - a common problem in
sci-fi TV and film is that the set designers don't always communicate
with the FX department, and vice versa, leading to interiors that are
too big to fit inside the ship they're designed for. It's usually close
but has to be fudged a bit in some way. Yes, the use of CG is starting
to eliminate that because one department can finalize their design and
send the file to the other department to continue their side of the job
but the problem still happens from time to time.
That being said, I will admit that there are plenty of designs that don't completely think the design through, however this is not always the designer's or FX shop's fault. Film and TV are collaborative efforts dictated by time and money above all else. Different people involved in those productions have different levels of regard for accuracy depending on their investment - both financial and emotional - in the production. As such, sometimes shortcuts will be taken to get the job done on time and/or budget. Other times it's a matter of someone in charge saying "I don't care - that's how I want it to look."
Either way, those are not the sci-fi hobbyists' fault. Our "job" is to re-create what we see - the same job as any other modeler who builds any other subject, which brings us to the color accuracy argument.
Among sci-fi modelers, we freely admit that there is more than one way to paint a given ship, just as military vehicles have been known to have more than one design of camouflage or base paint color depending on their era and location of service. The main issue with sci-fi ships comes down to one basic debate - "on screen" colors vs. "FX model" colors. "On screen" color is how the vehicle looked in the finished production. This can be and usually is affected by lighting, film processing anomalies and any number of video color balance settings both in the mastering of the DVD and the settings of your personal TV set. "FX model" color is how the model itself was painted which may have been done with different tones and shades specifically chosen to compensate for the above mentioned lighting, processing and color balance issues. Compounding both problems is that more often than not, several FX models were built in different sizes to accomplish different shots or multiple models of the same size were painted and detailed differently to represent this character's ship vs. that one's. That's more an issue of sorting out and double-checking your reference material - I'm sure you military modelers face the same issue of finding correct reference to how a vehicle looked during a particular time in its operational life. Different subject matter, same obstacle.
While there are many ways to argue in favor of one color choice over the other, it ultimately comes down to what the builder prefers. My personal logic has always been that it has to look "right" to my eye as I'm the one that's going to see it on my shelf every day. I don't build to make someone else happy, I build because I enjoy it. I don't live and die and center my life around how many awards I've won or not won over the years.