I was told that red and deep blue paints required gray (high hiding) primer. I used it before I painted my living room red. But how do I know which other paint colors do vs. just regular white primer?
E.g., I'm getting ready to paint some millwork coffee brown-ish. Should I use gray or is white fine?
I'd consider any color that's darker than the gray primer itself to be better suited to gray primer. It's not just about color hiding, but also edge exposure. If you prime with white and then paint a dark or bold color, it's possible that fringe white will show where you've masked, etc.
There are two color-driven reasons to use primer.
First is to make the surface consistent, so differences in color don't print through. So black screw-heads and white tape against blue insulating board don't show.
Second is to get the substrate closer to the final paint color, so you need fewer coats to actually get the color you were after.
I've had yellow jobs where it took 6 coats to cover gray primer with yellow. It was superb marine paint. But durable yellows don't have good covering power (opacity) since leads and chromates were banned. White primer covered better, but best was a custom mix of white and gray primer. Ultimately it was impractical to keep adding layers of yellow until it was completely opaque, so some print-through of primer color was inevitable, the key was to make it non-objectionable.
Look around at cars in the parking lot. Very few customers custom-order cars anymore, so those paint colors are chosen by the manufacturer, based on the cost of durable pigments. (using a non-durable pigment on a car with a 7 year warranty is out of the question). They do a few blues and reds at a loss, just to mix things up, but how often do you see yellow-based colors like green or orange?
A lot of deep base or transparent hues require a gray primer to achieve the desired color. For example, Sherwin Williams offers 6 different shades of gray primer (P1-P6). A specific shade of this gray primer is prescribed to different colors. If you were painting with a red or a deep green using a white primer then you would end up with a lot of extra work and money spent on your project. It would take multple unnecessary coats to cover and the color still wont match the collor you were going for. It mught also streak and picture frame really bad. Always speak with your painting rep to make sure you apply the right primer.
