Make different border color gallery items

in case you using SASS: you can use this snippet and change accordingly

$colors: red, orange, yellow, green, blue, purple;
$repeat: 20  // How often you want the pattern to repeat.
// Warning: a higher number outputs more CSS.

@for $i from 1 through $repeat {
    .gallery img:nth-child(#{length($colors)}n+#{$i}) {
         border-color: lighten(nth($colors, random(length($colors))), 20%);
    }
}

.gallery img {
    list-style: none;
    padding: 1em;
}

if you can’t work with SASS than you must use a javascript / jquery solution to achieve that. I posted a snippet that generates random color values. now you only need to find a logic to get the colors to the appropriate items.

var color = "#" + Math.floor(Math.random() * 0xFFFFFF).toString(16);

If you need further help, post some more code