|RYB Color Wheel from George Field's Chromatography (via Wikipedia)
Which is warmer? Red? Yellow? Depends on who you ask. Dianne Mize recently posted a video on YouTube on how to determine warm vs cool blues:
This video sparked a bit of debate because Dianne considers the warmest colour to be yellow as opposed to red, so for her, a warm blue leans to yellow and so that got me to doing a little digging...
The argument that Dianne makes is that the warmth is based on the observation of heat sources, such as flames, in which the hottest portion is an extremely pale yellow (even white) and that moves progressively cooler as it progresses through red and ultimately blue light. That's a pretty logical argument!
Having said that, if you fall into the camp that sees the colour wheel as basically the spectrum of visible light converted to a wheel, then the warmest end of the spectrum is red and the coolest end is blue, given the consistent assumption that blue is always the coolest. Isaac Newton documented this:
|Isaac Newton's circle courtesy of Wikipedia
That doesn't render Dianne's instruction wrong, you can apply the same principles to warm vs cool when looking at blue, just flip the direction. On the plus side, regardless of which camp you fall into, warm yellows will always lean red and warm reds will always lean to yellow. Blue side is always the cool side.
From my own observations, it seems to me that watercolourists seem to fall into the red is warmer camp. I say seem to because even there has no strong agreement, but a lot of artists when talking colour tend to indicate a bias to red. Jane Blundell has an article on warm blues that talks to this and her colour references and information are pretty close to gold standard for a lot of us. However, regardless of that, I think the key to thinking about it when it comes to blue, is to forget the concept of warm/cool and think about which way it leans: red or yellow. That's probably more useful from a mixing perspective anyways.
So, where do I land? I land on the red is warmer because I mentally wrap the spectrum of visible light to generate the wheel in my thoughts and so I see the red end as warmer. Also influencing my thinking, is my background in photography. Reddish light is considered warm and bluish light is considered cool, this is a function of the physics behind the colour temperature range (and not tied to Kelvin comparison to heat). So, because of this, the idea of red being warm just sticks with me.
In any case, warm vs cool is a construct, or a tool, to help you pick better colours for mixing. If you just think about relationship to the next nearest primary, then you can use that to best determine the mixing choices to maximize vibrancy and reduce mud.