Memel 1923 Occupation Sn:n26 / Sg:26 (?)

I found this 500m Memel Lithuanian Occupation stamp and it appears to be gold in color (the uploaded image has close color accuracy) [or perhaps the 20m Sn:N19 / Sg:19 yellow on yellowed paper] rather than the standard Sn:N26 lilac / Sg:26 purple, also shown for reference. Does anyone have access to alternate catalogs or information that describes this item?

By the look of the paper of your stamp, it has been badly stored in the past. The gold color is a result of fading of certain pigments or chemical changes ("changeling") or a combination of both. I think it is mostly fading as I've seen lilac/purple printing do pretty much the same thing when left out in the sun.

Unfortunately, I don't have an example to show, but see the surcharged version here:
http://goscf.com/t/18758
While the image is too bright, you can see a change of color happening from bottom to top, on its way to turning more yellowish.

What are those lines, outside the design bottom and top?

Looks to me like it has been stored in a "Magnetic" Photo album perhaps.
Tiny lines of adhesive gum.

The similar effect is in the "background of the normal example.

EDIT: I believe the effect is caused by relatively thin inking on paper that has a strong enough weave to appear somewhat horizontally ribbed. Other values using this design also show this though not every stamp does.

Remember this is a relatively small-size stamp . The wax lines applied to "magic" photo album pages from what I've seen are a lot wider and wider apart than what I'm seeing here.