A lot of experts offer all sorts of helpful hints for checking out your ignition system based on the color or length of the spark you'll see when you hold the coil's center-lead near a ground while cranking the engine.
Virtually all of that 'expert' advice is bullshit.
Faulty color perception is a sex-linked trait that effects mostly males and is surprisingly common. (There is a form of monochromatic visual abnormality that effects both men and women but in those cases, which are extremely rare, they see no color at all, everything is in shades of gray.)
Show a classroom of would-be mechanics a color slide of a spark... or even a picture of a car(!) and their descriptions of the color will vary wildly. Bluish white, sez one. Except it was sorta reddish white to the guy next to him and purplish white to the guy over there.
The point here is that before you can use color perception as a diagnostic tool you will have to calibrate your 'instrument' :-)
Second point: What gives the spark its color to begin with?
If the spark took place in a vacuum tube, it will always emit the same
spectrum. But if the spark occurs in air, the color will be effected by the
gases... and contaminants... that make up the ambient atmosphere. Damp day?
Different color than a dry day. Inside a shop with lots of exhaust gases floating
around? Different color. Spray booth next door? Different color.
The atmosphere (or lack of it) in which the spark occurs also effects the gap
the spark can jump, as does the surface smoothness of the two surfaces. If you
have a rough surface you can usually induce a relatively low-voltage spark to
jump the gap. Two polished surfaces, you'll need a higher voltage to initiate
the spark. (Why do you think it's unwise to stand under a tree during a
thunder storm? :-)
The bottom line is that visually checking to see if you've got a spark tells you only
that the induction coil and points are working. Any effort to evaluate the
spark voltage based on color or the length of the spark will produce a wide
variation of conclusions even among skilled mechanics.
Here's a good basic rule for mechanics: Unless someone is shooting at you,
don't guess. Measure.
Once you've learned to do something the right way and have done it for a while,
your store of experience will become a valid guide. But don't assume your
perception of a given phenomenon is universally shared. Perceptions are based
on your senses. Color perception in males varies widely and a surprising
number of youngsters today are functionally deaf to certain frequencies, unable
to hear some mechanical problems that are clearly evident to others. In the
same vein, smokers and guys doing nose-candy often have no sense of smell at
all, unable to smell a burned clutch or overheated engine that may be evident
to you from a block away.