When you say "specific colour", do you mean like a solid colour (i.e. the thumbnail shows only the colour blue and no hint of the image) or that the image has a blue-ish tint?
is there a way to make character's thumbnails / avatars be a specific color like blue but when hovered over it shows the actual color?
In that case, you can try using filter.
.user-characters .img-thumbnail > img {
transition:ease-in-out .25s;
filter:sepia(100%) saturate(210%) brightness(70%) hue-rotate(150deg);
}
.user-characters .img-thumbnail:hover > img {
filter:none;
}
Unfortunately, there's no really "easy" way to pick-and-choose specific colours for the tint without possibly affecting the overall thumbnail itself, so you'll have to fiddle around with the filter values until you get the desired look.
hue-rotate is the one that will change/ affect the colour the most (i.e. if you want to change the colour to yellow, green, etc), whereas saturate and brightness are more for when you want to fine-tune adjust the colour. It's important to keep sepia at 100% though for a consistent colour throughout; otherwise, the filter will change depending on the colours of the individual image itself.