Often “warmer” in PS is referring to making the picture more vibrate. Sometimes it doesn’t involve adding color but in turn making it more dynamic. Try this next time you’re trying to “warm” a picture up. Open the layer in Lab Color (This is right next to RGB & CYMag settings) then open up Curves. Now before effecting anything, open up the drop down menu and select A. See the line heading diagonally across the grid? Take both corners and bring them across toward the middle of the column, do not go down or up with either corner. Bring both sides in about half way through the grid (the first block of the grid). Do this for both pull down tabs A & B, do not adjust lightness. When you’re done you can change it back to RGB and continue to save back to .jpeg if you like or save for web still works. I made an example just in case you wanted to see what it would look like.
Before
After
See how I didn’t add any color, but it still brought more out? This is what I would consider warming up a picture in terms of using Photoshop.