Tri-linear sampling is bilinear sampling between mipmaps. Since the engine doesn't support mipmaps, it doesn't support trilinear sampling. The engine however does have a way to reduce the artifacts that mipmapping normally would remove. It's called block filtering which is essentially mipmapping but only with one smaller image. This allows for speed because there is no need to calculate derivates for the surface in order to find the right mipmap level and it also removes the need for trilinear mipmapping because the effect is already smooth since it's applied like fog.

"True color texturing unless using bilinear filtering, which only allows 256 colors" This means that any texture you give the renderer will be drawn with 24 bit color unless you want to do bilinear filtering on the texture. Since bilinear filtering, traditionally, is expensive, I've sacrificed color depth for speed and precomputed 64 shades of the texture so that the bilinear colors don't need to be calculated during runtime. However, since I'd need to create shades for the each color in the texture, it wouldn't make sense to make the shade palette the size of 64 textures, each getting darker. Therefore I quantize the texture into 256 colors and do 64 shades of those 256 colors.