You can use custom skins in game. The coloured skins are so noobish, if you use them you will get laughed at. Bright coloured skins are only really useful if you are using a material wallhack anyway.

You can use custom skins on the server, just beware that some of them look absolutely terrible. I used to use a few custom skins, but the scrim servers forced me to use the standard skins, and the gun sounds glitched out because of this, so it was horrible and I went back to the normal skins. The only skin I use now is this really well textured skeleton glove skin.

It's probably best to keep your default player skins, but change weapon skins and gloves/sleeves if you want to