如果在Android上使用webrtc时出现黑屏或绿屏，并且使用UV4L流，请尝试以下解决方法：

1. 确保正确初始化和配置webrtc库。

PeerConnectionFactory.initializeAndroidGlobals(context, true);
PeerConnectionFactory.Options options = new PeerConnectionFactory.Options();
DefaultVideoEncoderFactory defaultVideoEncoderFactory = new DefaultVideoEncoderFactory(
    rootEglBase.getEglBaseContext(), true, true);
DefaultVideoDecoderFactory defaultVideoDecoderFactory = new DefaultVideoDecoderFactory(
    rootEglBase.getEglBaseContext());
PeerConnectionFactory.initialize(
    PeerConnectionFactory.InitializationOptions.builder(context)
        .setOptions(options)
        .setVideoEncoderFactory(defaultVideoEncoderFactory)
        .setVideoDecoderFactory(defaultVideoDecoderFactory)
        .createInitializationOptions());

2. 确保正确设置和配置SurfaceView作为视频显示区域。

SurfaceViewRenderer localRenderer = findViewById(R.id.local_renderer);
localRenderer.init(rootEglBase.getEglBaseContext(), null);
localRenderer.setScalingType(RendererCommon.ScalingType.SCALE_ASPECT_FILL);
localRenderer.setMirror(true);
localRenderer.setZOrderMediaOverlay(true);
localRenderer.setVisibility(View.VISIBLE);

3. 在创建VideoTrack时，确保正确设置视频源。

MediaConstraints videoConstraints = new MediaConstraints();
videoConstraints.mandatory.add(
    new MediaConstraints.KeyValuePair("maxWidth", "640"));
videoConstraints.mandatory.add(
    new MediaConstraints.KeyValuePair("maxHeight", "480"));
videoConstraints.mandatory.add(
    new MediaConstraints.KeyValuePair("maxFrameRate", "30"));

VideoCapturer videoCapturer = UV4LCapturer.create(
    uv4lServerUrl, uv4lUsername, uv4lPassword, videoConstraints);

VideoSource videoSource = peerConnectionFactory.createVideoSource(videoCapturer.isScreencast());
videoCapturer.initialize(
    surfaceTextureHelper, context, videoSource.getCapturerObserver());
videoCapturer.startCapture(
    videoConstraints.mandatory.get("maxWidth"),
    videoConstraints.mandatory.get("maxHeight"),
    videoConstraints.mandatory.get("maxFrameRate"));

VideoTrack videoTrack = peerConnectionFactory.createVideoTrack("videoTrack", videoSource);
videoTrack.addSink(localRenderer);

确保正确设置UV4L服务器的URL、用户名和密码，并根据需要调整视频的最大宽度、高度和帧率。

这些解决方法应该能够帮助您解决Android上使用webrtc和UV4L流时的黑屏或绿屏问题。请根据您的具体情况进行适当的调整和修改。