Custom data in info.dat

Custom data can also be placed on entire levels (songs) and on individual difficulties. For example:

// ...
"_songFilename":"song.ogg",
"_coverImageFilename":"cover.png",
"_environmentName":"DefaultEnvironment",
"_customData":{
    "_contributors":[
        {
            "_role":"Kirb",
            "_name":"Kyle 1413",
            "_iconPath":"derp.png"
        },
        {
            "_role":"Lighter",
            "_name":"Kyle 1413 The Second",
            "_iconPath":"test.png"
        }
    ],
    "_customEnvironment":"",
    "_customEnvironmentHash":""
},
"_difficultyBeatmapSets":[
    {
        "_beatmapCharacteristicName":"Standard",
        "_difficultyBeatmaps":[
            {
                "_difficulty":"Easy",
                "_difficultyRank":1,
                "_beatmapFilename":"Easy.dat",
                "_noteJumpMovementSpeed":0.0,
                "_noteJumpStartBeatOffset":0,
                "_customData":{
                    "_difficultyLabel":"",
                    "_colorLeft":{
                        "r":0.013660844415416155,
                        "g":0,
                        "b":0.07069587707519531
                    },
                    "_colorRight":{
                        "r":0.0014191981941151946,
                        "g":0.14107830811467803,
                        "b":0.07064014358987808
                    }
                }
// ...

This data can be accessed by casting a BeatmapData to CustomJSONData.CustomBeatmap.CustomBeatmapData. The first "_customData" section (the one for the whole level) is provided as levelCustomData, and the second (the one for the Easy difficulty) is provided as beatmapCustomData.
The per-level custom data section is also available from a StandardLevelInfoSaveData by casting to CustomJSONData.CustomLevelInfo.CustomLevelInfoSaveData and accessing the customData property, or from an IPreviewBeatmapLevel using the extension method CustomJSONData.Extensions.getLevelCustomData.