I assume then, the hooves are white so you can see the red.
A lot of times, horses will react to a "sugar overload" in their system and it will manifest in the hooves. If there was a weird change in weather, the grass can produce more sugar than normal. Lots of factors that go into that, but that's the simple version.
When there is an unsual sugar spike in the horses' body, it can cause mild inflammation in the hooves (that in worse cases manifests as laminitis/ aka founder) but it doesn't have to cause lameness. If your horse has dark feet you likely won' t notice the color (duh) but sometimes you'll notice a "growth ring" that looks like the hoof grew thicker for about 1/4-1/2 inch. It's basically a mild bout of laminits that didn't do any real harm. Once that red is there, it has to grow out.
Another version of this you'll see sometimes is red spots when the hoof is freshly trimmed. It looks like a fresh bruise but it is made when the hoof horn is made and grows out. Horn doesn't let oxygen pass through freely and once it's formed, the blood vessels no longer reach it. It stays fresh looking for that reason, but can be weeks/months old
It's a good sign to watch for to catch metablolic issues in horses, esp if they are fat. If you start seeing a lot of red marks showing up in the light hooves, or small growth rings, then you might consider a diet change. If it's once in a blue moon that it occurs, it's not a big deal as long as the horse isn't hurting from it.
A side note, when you see those growth rings..as it grows out to ground level, that particular section of wall is weaker than normal and will be more prone to chipping or separating, so be esp. sure to keep up on your trimming maitenance.