Вы устанавливаете selectionBackgroundColor
в строку rgba, чтобы придать цвету значение непрозрачности.
Пример:
fabric.Group.prototype.selectionBackgroundColor = 'rgba(45,207,171,0.25)';
var canvas = new fabric.Canvas("canvas");
fabric.Group.prototype.selectionBackgroundColor = 'rgba(45,207,171,0.25)';
canvas.add(new fabric.Rect({
width: 50,
height: 50,
left: 50,
top: 100,
fill: 'green'
}));
canvas.add(new fabric.Rect({
width: 60,
height: 60,
left: 250,
top: 100,
fill: 'red'
}));
canvas.on('object:selected', function (e) {
e.target.transparentCorners = false;
e.target.borderColor = '#cccccc';
e.target.cornerColor = '#0CB7F0';
e.target.minScaleLimit = 2;
e.target.cornerStrokeColor = '#0CB7F0';
e.target.cornerStyle = 'circle';
e.target.minScaleLimit = 0;
e.target.lockScalingFlip = true;
e.target.padding = 5;
e.target.selectionDashArray = [10, 5];
e.target.borderDashArray = [10, 5];
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.7.0/fabric.min.js"></script>
<canvas id="canvas" height="400" width="400"></canvas>