“Foreign key constraint is incorrectly formed”が出た時

これで治った

  Schema::create('colorset_fixture_relation', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->unsignedBigInteger('fixture_id');
            $table->unsignedBigInteger('color_id');
            $table->foreign('fixture_id')->references('id')->on('fixtures');
            $table->foreign('color_id')->references('id')->on('colors');
            $table->timestamps();
        });