Hello there. Primer is only needed for specific purposes. Say for example they had painted the drywall with a oil base paint and you wanted to transition to a water base, then applying a transition primer first would be a good idea. Or for example, say the drywall had just been finished (as in, just coated with drywall compound) then applying a primer first before your finish paint coat is a must. In both examples the reason for applying the primer is different, however, fundamentally they are for the same reasons, which is to ensure adhesion. Poor adhesion means the paint will not stick or rather it will not stick as the manufacturer intended the product to thus, reducing longevity, if not causing complete failure of the coating.

In your particular case, the wall has already been painted, ie. the drywall should have already been primed prior to them applying the finish coat of paint. Thus, there is no need to prime in this circumstance unless for very specific reasons, like for example, as explained in my first example. Although oil based paints are presently rarely used so I do not believe that to be a likely scenario.

Sanding a surface prior to painting, on the other hand, is usually considered standard textbook protocol to improve adhesion, although it is very rare for anyone even professionals to sand a surface prior to appying a paint coat especially on a wall surface. In-fact it can be more dangerous then beneficial. Let me explain. Again sanding prior to painting has to do with improving adhesion. However, the properties of many good quality paints compensate well should you choose not to sand. So, if an extremely fine grit sandpaper is not used or a surface oversanded, then when you apply the paint coating, the paint properties (viscosity) will prevent the paint from being able to fully fill the fine scratches created by the sandpaper thus, creating a very unsightly surface after painting. The goal is to see a smooth consistant surface not a scratched one. Sanding is typically reserved for situations such as surfaces with extremely high gloss paint finishes. The reason for this is on a microscopic level, the higher the gloss the more 'smooth' the surface will appear. Thus, without surface 'bumps' the paint does not have much to grip to and so, sanding the surface makes up for that loss of grit.

Thanks for all the feedback. So from what I'm gathering I shouldn't have to apply Primer or Sand the wall before applying nee paint over the existing. Does this change at all if I'm going from a light colored wall (White) to a darker colored wall (Turquoise)?

Glad to help. Yes, neither is mandatory in this situation. What is however, is ensuring the surface is clean. That may seem like a given although, I want to point out that you may use a product called 'TSP' which is an industry standard for surface preparation. It is typically more so necessary if there is grease, oils, or other grime that may require more then a wet rag to clean.

Regarding the more darker color over light, in-fact it works the opposite. If you have a darker and/or brighter-deeper color (such as a red), that you intend to cover, then sometimes it is advisable to prime first otherwise you may need to put more then 2 coats (industry standard) of finish coat in order to cover it. In the case of a darker color being put over a lighter one, you have the upper hand since there is more pigments in your paint that will assist with coverage.

TSP cleans well but I would not use it on drywall because it has to be thoroughly rinsed off. If the walls are clean of any heavy oils or grease - which since they are new they probably are - you don't need to clean them.

A better choice for wall cleaning is Dirtex. No rinsing.

@graywings123 Interesting. I will look into Dirtex as I am always interested in superior performing products. I am interested to know how long it has been around for. However, I would also not use TSP for drywall. I was suggesting it in context for the this project, which as I understood, the drywall to have already been primed and painted.

"I'm gathering I shouldn't have to apply Primer or Sand the wall before applying new paint over the existing. Does this change at all if I'm going from a light colored wall (White) to a darker colored wall (Turquoise)?"

YES! And the primer will be gray.

If you are using a Sherwin-Williams paint, there is a shade of gray primer recommended for the paint as described here. I was having a terrible time repainting a white wall with SW 6914 "Eye Catching" until I primed the wall with Sherwin-Williams Multi-Purpose.

I like Behr paints (which offers many turquoise paints) and Zinsser 1-2-3 primer which is available in gray.

FYI, you can always get the primer tinted to the color of the wall paint you plan to use for a 2-in-1 step. Second the vote for Behr paints, good quality.