Honestly, I tend to use whatever miso I have in my fridge. Sometimes what I have is yellow, and other times it may be red. One may be a bit stronger than the other, but I truly don't notice it. Unless, it were in a miso soup where miso was the star ingredient.

One other note about miso if you're buying from a Japanese store.
shiromiso is "white miso".

Some USA made miso is excellent..very excellent. "Miso Masters" from the Pacific NW has some great misos. I love their red Miso. It's usually available at health food stores instead of ethnic store. It's far better than any imported miso I've had.

Miso keeps very well the 'fridge. So, it's worth the price.
It's fermented and might get slightly more salty tasting, but I like that. It can keep a year or more. I've had a tub of red for two years, and it's still wonderful and rich. Just seal it well and maybe press down some plastic wrap on top keep air off the surface.
If it does harden on the surface..just scoop that out..stuff under it is fine.

I prefer red to white miso...even in soup. But then again I like the stronger salty taste.
Red is used mostly for autumn and winter dishes due to it's stronger taste.

If that's all you have on hand, you could thin it out with 1/2 of the miso called for and 1/2 tofu whisked in a blender.

@merrill
We share the same last name. Does your family come from NC in history?

I'd use white or yellow miso -- red would probably be a little strong. And yes, agave nectar and agave syrup are the same!