Yep it's just the depth of the wort and probably related to the camera flash etc, all the flasks were filled with the same wort.
The wort in the 2lt flask looks considerably darker than the other 2.
Is this just due to the depth of the wort, or a different batch?
Yes, if I only had one slant I'd be much more careful, use the procedure you mentioned and probably a smaller amount of wort for the first step. However I generally make 3 or 4 slants of each yeast I have, this seems to work well for me in that I usually have 1 left after about a year when it's time to reslant that strain.
I did cringe at the addition of wort to the slant and scraping the yeast off, it's a waste of a good slant IMO.
Preferred method would have to be to take, asceptically, a small amount of yeast from the slant with an innoculating loop and add that to your first bit of wort. This will allow you to retain the slant for future brews/comparison tests and allow you to maintain a pure culture strain for a longer period of time.
The above comment is based entirely on there being just the one slant, if you've got a bunch of the same yeast on a shed load of slants then it's a non issue for you.
I'll be following your procedure with the (one) commercial slant I have here, someone has said that they have been reusing the same slant - just taking a small amount of yeast each time - for 4 years now.
Test tubes of yeast would require less preparation, but I think the yeast stored on wort/agar is a little more stable and once made they are easy to handle and store.
I've always just used test tubes of yeast. What would be the pros/cons of slants over a test tube of yeast?
You can also create a slant from a small sample of 'virgin' generation yeast - from a streak plate for example - and it's easy to reslant old samples so that the yeast always remains viable, with test tube storage you'd need to grow and split the yeast each time you needed to reculture it.
Splitting new packs of Wyeast into test tubes via Tony's method (or similar) is much easier (and maybe better) than using slants, but it really 'only' lets you split the pack into a limited number of test tubes, once those are used you'll need to buy a new pack.
With slants (and streak plates) you should be able to 'save' that particular strain of yeast for an indefinate period.
Personally, I tend not to over-complicate things in this regard, so I make some big assumptions in regard to the cell count.
So how many yeast cells do you estimate you have in the final starter?
I assume that since I am using small steps, a stir plate and a healthy dose of yeast nutrient, at each stage I'll get maximum growth from the yeast.
Then I let someone Else's maths do the work for me, and see what size starter is recommended by the MrMalty calculator (assuming 1 pack of yeast and maximum growth).
Assuming again that the second-to-last-step of the starter process should have about the same number of yeast cells as commercial pack/vial of yeast (I don't recall where the basis for that theory came from), I base my starter volume on what the calculator suggests - usually rounded up to the next convenient size container I have.