graphicxtras.com > Photoshop gradients > Photoshop gradients and the creation of contours using the outer glow (or inner glow) layer effects
You can create this effect by using a preset made of multiple stops and added as an outer glow to any selected shape etc via layer effects / styles dialog in the layers menu. You can change the technique to precise and set the opacity and blend mode and spread and size as required. Set the blend to normal.
You can stop at this point if you want a colourful contour for the shape but if you want a black and white contour or perhaps different colors, rasterize the layer effect via the layer menu rasterize layer effects command. You can then go to the image menu and adjustments and threshold to reduce the image to black and white contours. Below is a single ellipse with a single outer glow and rasterized and then threshold adjustment added
Below you have the original layer with the glow all rasterized and remaining as a layer and then a new outer glow is added to the artwork using the same preset. It is a pity that the outer glow as well as all the other features in the layer menu cannot have a multiple / copies / repeat feature (many apps do but PS for some reason does not). Note that the circles are a little cruder. This is not so notable if you use a more unusual vector path than a basic ellipse
In the example below, a cross has been added as a layer (a combination of two rectangles and merged) and the layer style has been rasterized via the layer menu and then another glow with the same preset has been added and then flattened and then the threshold adjustment has been added. You do not need to flatten the artwork, as you could just add an adjustment layer. You can also turn the layer into a smart object and then add a new smart filter / adjustment 'threshold' and the threshold will then only be added to the object. Set the adjustment setting as required to have more or less of the lines included
You don't have to use the threshold, you can leave them in color
You can add multiple vectors with multiple glows and the glows can be made up of multiple color stops. You can turn the artwork into a smart object via the layer menu and then add a hue / saturation etc adjustment and you can rapidly create a suoer colorful background by creating multiple copies of the object with different hue and saturation settings (or any of the other colorful adjustments such as LUTs)