Simple editor script that will create a 4x4 texture of the specified color. Useful for prototyping with sprites.

Editor\MakeSpriteTexture.cs

using UnityEngine;
using UnityEditor;


public class MakeSpriteTexture : EditorWindow
{
	[MenuItem("Assets/Create Sprite")]
	static void Init()
	{
		MakeSpriteTexture window = GetWindow<MakeSpriteTexture>("Create Sprite");
		window.Show();
		Rect winPos = window.position;
		winPos.width = 210;
		winPos.height = 100;
		winPos.position = new Vector2(Screen.width * 0.5f, Screen.height * 0.5f);
		window.position = winPos;
	}

	private Color color = Color.white;
	private string relPath = "_Sprites/Colors";
	void OnGUI()
	{
		GUILayout.Label("Select color the press create.", EditorStyles.boldLabel);

		relPath = GUILayout.TextField(relPath);

		color = EditorGUILayout.ColorField(color);

		if (GUILayout.Button("Create"))
		{
			EnsurePath(relPath);
			CreateTexture();
		}

		//GUILayout.Label(string.Format("({0}, {1})", position.width, position.height));
	}


	void EnsurePath(string path)
	{
		string fullPath = path;
		if (fullPath[0] != '\\' || fullPath[0] != '/')
			fullPath = "/" + fullPath;

		path = path.Replace("\\", "/");

		fullPath = Application.dataPath + fullPath;
		string[] folders = path.Split('/');

		if (!AssetDatabase.IsValidFolder("Assets/" + path))
		{
			Debug.LogFormat("Folder '{0}' is not valid. Creating path: {1}", fullPath, path);

			string parent = "Assets";
			for (int i = 0; i < folders.Length; i++)
			{

				if (i > 0)
					parent += "/" + folders[i - 1];

				string guid = AssetDatabase.CreateFolder(parent, folders[i]);
				string newFolderPath = AssetDatabase.GUIDToAssetPath(guid);
				AssetDatabase.Refresh();
				Debug.LogFormat("parent: {0}  folder: {1}  newFolderPath: {2}", parent, folders[i], newFolderPath);
			}

		}
		else
		{
			//Debug.LogFormat("Path: '{0}' is valid", path);
		}
	}

	void CreateTexture()
	{
		Texture2D tex = new Texture2D(4, 4);
		for (int i = 0; i < tex.width; i++)
		{
			for (int j = 0; j < tex.height; j++)
			{
				tex.SetPixel(i, j, color);
			}
		}
		tex.Apply();
		byte[] bytes = tex.EncodeToPNG();

		string fileName = color.ToString().Replace("RGBA", "") + ".png";
		string dbPath = "Assets/" + relPath + "/" + fileName;
		string systemPath = Application.dataPath + "/" + relPath + "/";

		System.IO.File.WriteAllBytes(systemPath + fileName, bytes);
		Object.DestroyImmediate(tex);
		AssetDatabase.Refresh();

		TextureImporter tImport = (TextureImporter)AssetImporter.GetAtPath(dbPath);
		if (tImport == null)
			Debug.LogError("tImport sucks");
		else
		{
			tImport.textureType = TextureImporterType.Sprite;
			tImport.mipmapEnabled = false;
			tImport.maxTextureSize = 32;
			tImport.filterMode = FilterMode.Point;
			AssetDatabase.ImportAsset(tImport.assetPath, ImportAssetOptions.ForceUpdate);
		}
	}
}