within the project page, no. generally, no.
it’s possible to specify the color of the API tiles, but the color would apply to the whole set of observations, not for individual markers in the same set based on some variable. there’s no standard functionality designed to allow you to change the size of the markers in API tiles. you could do some sneaky things to change the size of the markers in the API tiles (so that different sets could have different sized markers), but the marker size would be the same within the same set, and it would be convoluted and imprecise procedure and probably not worth the effort.
the red and green are 2 different layers. the way they’re being delivered / rendered is completely different. you didn’t do anything to cause the page to display the two different layers. i just think that’s how the page displays observations.
traditional projects are sort of an old concept, and i think the design of the traditional project pages is relatively old. the green markers are probably how the page originally rendered observations on the map, but that way of rendering observations doesn’t scale very well (when you have a lot of observations). so probably at a later date, as number of observations exploded, they realized they needed a more scalable way to map observations. so they added the red markers. but they never removed the green markers for whatever reason. so that’s why you see the mix of the 2 different kinds of markers.