[Seany (OP)]

When you draw on a balloon (not inflated) with a black marker pen, it comes out.. very BLACK.

Then when you blow it up, the black lines turn grey.

Why is this? Obviously it's to do with it stretching.. But..

[lightarrow]

When you draw on a balloon (not inflated) with a black marker pen, it comes out.. very BLACK.

Then when you blow it up, the black lines turn grey.

Why is this? Obviously it's to do with it stretching.. But..

The colour grey is nothing else than black seen in a light background.
When you inflat the balloon, the ink particles spread, increasing their distance, so their percent area relative to the background decreases. This is perceived as grey.

Make this experiment: draw many little black points with a pen on a white paper, all together in a group, let's say they are separated about 1 or 2 mm one to another, then look at this "patch" from some distance: when the distance is such that you cannot perceive every point any longer, you will start to see a grey patch.

[Seany (OP)]

Ohhhh.h...... But is this because, your eye's are playing tricks on you?

[lightarrow]

Ohhhh.h...... But is this because, your eye's are playing tricks on you?

In my opinion, it's not a trick, but simply the fact that our eye, considered as optical instrument, has a limited possibility to resolve a black point, so, farther than a certain distance, colour informations are mixed up, so the white (of the background) get mixed up with the black (of the points) and this comes out to be grey.

[Pumblechook]

I would think that you are extending the area over which the ink is spread so the colour 'saturation' is reduced.   Just like the difference in colouring a small piece of paper and colouring a much larger piece with the same amount of ink. 

[lightarrow]

I would think that you are extending the area over which the ink is spread so the colour 'saturation' is reduced.   Just like the difference in colouring a small piece of paper and colouring a much larger piece with the same amount of ink. 

Yes, but this is equivalent to what I wrote. The saturation is the degree of mixing between a pure colour and the white: pure colour --> 100% saturation; white --> 0% saturation; half pure colour half white --> 50% saturation. When you see a white paper with a thin layer of ink on it, white light of the environment is reflected off the white paper so you see the colour of the ink with some percent of white light added; the thinner the ink layer, the more percent of white light, the less saturated the colour.

[rhade]

Grey is just black with reduced opacity. Try playing around with Desk Top Publishing or illustration software and you'll see what I mean. However, I think the issue here is akin to pixilation. Just as a printed image, blown up, breaks down into little dots (pixels) when the size is increased, so, when the balloon stretches, the spots of ink that make it up are being moved away from each other- effectively, pixilation is occurring.