I was following a tutorial in order to learn nodes for 2.8. When this line turned red and made the entire thing pink (missing textures). Does anyone know how to prevent this?
In the latest builds of Blender 2.8, the node editor has been enhanced to highlight some links in red to indicate that they are invalid connections. In this particular case, you're using a Mix Shader node to mix colors - you should instead be using a MixRGB node for this; the Mix Shader should only be used to connect to Green sockets.
In general, you should only connect 'like for like' - ie, green to green, grey to grey, yellow to yellow, etc. There are exceptions to this and Blender can automatically convert between some types - eg, connecting a Scalar (Grey) socket to a Color (Yellow) will automatically generate a greyscale color. Similary, connecting a Color (Yellow) to a Scalar (Grey) will average all the color channels together as
(r+g+b)/3 to use the result.
However, Green represents a shader output - which is more than just a surface color and cannot be automatically converted - so connecting one of those to another color is not valid. Previously Blender would allow this without warning and simply produce an invalid result, but the Node Editor has now been enhanced to warn of such invalid connections by highlighting the link in Red.
On The Red Line Put The "Shader To RGB" Node (Eevee Exclusive)
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.