DebugRenderer.cs

public class DebugRenderer : MonoBehaviour
{
    private bool _enabled = true;

    static Material lineMaterial;
    static void CreateLineMaterial()
    {
        if (!lineMaterial)
        {
            // Unity has a built-in shader that is useful for drawing
            // simple colored things.
            Shader shader = Shader.Find("Hidden/Internal-Colored");
            lineMaterial = new Material(shader);
            lineMaterial.hideFlags = HideFlags.HideAndDontSave;
            // set color
            lineMaterial.SetColor("_Color", Color.white);
            // Turn on alpha blending
            lineMaterial.SetInt("_SrcBlend", (int)UnityEngine.Rendering.BlendMode.SrcAlpha);
            lineMaterial.SetInt("_DstBlend", (int)UnityEngine.Rendering.BlendMode.OneMinusSrcAlpha);
            // Turn backface culling off
            lineMaterial.SetInt("_Cull", (int)UnityEngine.Rendering.CullMode.Off);
            // Turn off depth writes
            lineMaterial.SetInt("_ZWrite", 0);
        }
    }

    public void Update()
    {
        if (Input.GetKeyDown(KeyCode.F11))
            _enabled = !_enabled;
    }

    public void OnRenderObject()
    {
        if (!_enabled || Camera.current != Camera.main)
            return;

        CreateLineMaterial();
        // Apply the line material
        lineMaterial.SetPass(0);

        GL.PushMatrix();
        // Set transformation matrix for drawing to
        // match our transform
        // GL.MultMatrix(transform.localToWorldMatrix);

        var collisionGroup = Contexts.sharedInstance.logic.GetGroup(LogicMatcher.AllOf(LogicMatcher.Collider));
        var collisionEntities = collisionGroup.GetEntities();

        GL.Begin(GL.LINES);
        GL.Color(Color.red);

        Vector3 ConvertToVector(FixedVector2 vector)
            => new Vector3(vector.x.ToPreciseFloat(), vector.y.ToPreciseFloat(), 0f);

        /*
            * version that uses the bounds, not rendering floors. not sure what's preferable
        void Vertex3(FixedVector2 vector)
            => GL.Vertex3(vector.x.ToPreciseFloat(), vector.y.ToPreciseFloat(), 0f);

        foreach (var entity in collisionEntities)
        {
            var bounds = entity.collider.value.bounds;
            Vertex3(bounds.bottomRight);
            Vertex3(bounds.topRight);
            Vertex3(bounds.topRight);
            Vertex3(bounds.topLeft);
            Vertex3(bounds.topLeft);
            Vertex3(bounds.bottomLeft);
            Vertex3(bounds.bottomLeft);
            Vertex3(bounds.bottomRight);
        }
            */

        foreach (var entity in collisionEntities)
        {
            var edges = entity.collider.value.edges;
            foreach (var edge in edges)
            {
                GL.Vertex(ConvertToVector(edge.start));
                GL.Vertex(ConvertToVector(edge.end));
            }
        }

        GL.End();

        var velocityGroup = Contexts.sharedInstance.logic.GetGroup(LogicMatcher.AllOf(LogicMatcher.Position, LogicMatcher.Velocity));
        var velocityEntities = velocityGroup.GetEntities();
            

        GL.Begin(GL.LINES);
        GL.Color(Color.green);
        foreach (var entity in velocityEntities)
        {
            var v = ConvertToVector(entity.position.value);
            GL.Vertex(v);
            GL.Vertex(v + (ConvertToVector(entity.velocity.value) / 3));
        }

        GL.End();


        GL.PopMatrix();
    }
}

Updated to use different Colors

Updated to Show Hitboxes of Floor (old version kept as a comment, not sure what's better) & removed weird Artifacts in the background (caused by the class rendering to the Background Paralax Cam)