Graphic Designers
Graphic Designers
[Cueball stands in a lightly adorned room of a house, facing an open doorway. Each surface is painted an almost imperceptibly different shade of off-white, except the floor which is white. There is a rug, a couch with a pillow (white) and a book. There are two windows, in the right there is a potted plant. Knit Cap stands in the open doorway, as if about to enter the house, one foot at the threshold, but not on the floor inside. Cueball is reaching towards the doorway, inviting Knit Cap to enter.] Cueball: Come on in! We just repainted. Knit Cap: I... can't.
[Caption below the panel:] If you want to set up a vampire-style barrier to keep graphic designers from entering your house, just paint every surface a slightly different shade of off-white.

Being presented with visual information that is just not quite right is known to cause feelings of unease and revulsion, particularly when presented with CGI human faces, a concept known as the uncanny valley .
In the title text, it mentions a contingency against the designer managing to actually overcome this disgust. In this case, Cueball sets up a second way to troll his graphic designer friend using some picture frames, a level , and a protractor that can measure increments of less than a degree. Cueball can then skew his picture frames by an extremely small amount, noticeable only to the designer friend, to disgust him even further — similar to the effect of bad kerning . This could thus be applied like the use of crosses or garlic , which vampires are famously repulsed by.
Although the window ledges are slightly inclined, falling subtly from left to right, it is unlikely this is a deliberate aspect of the room so much as a side-effect of Randall's imprecise stick-figure drawing style.
True to the comic's joke, Randall has actually colored each segment of the comic differently to each other, even though normal persons would just perceive all walls as slightly gray (off-white). 
The hexadecimal color codes are:
Whilst this subtle difference may be undetectable to humans without a graphic design qualification, it can be made clearer by increasing the saturation value of the image, as shown in this rendering with an exaggerated color saturation .
[Cueball stands in a lightly adorned room of a house, facing an open doorway. Each surface is painted an almost imperceptibly different shade of off-white, except the floor which is white. There is a rug, a couch with a pillow (white) and a book. There are two windows, in the right there is a potted plant. Knit Cap stands in the open doorway, as if about to enter the house, one foot at the threshold, but not on the floor inside. Cueball is reaching towards the doorway, inviting Knit Cap to enter.] Cueball: Come on in! We just repainted. Knit Cap: I... can't.
[Caption below the panel:] If you want to set up a vampire-style barrier to keep graphic designers from entering your house, just paint every surface a slightly different shade of off-white.
