In Chat Groups Guilds you can customize the color, font, size and style of message types. This can be done in the ChatGroupGuildsRichText data table, this is found in the other folder in the content browser.

You can select the chat type row and in the Row Editor you can set that message types text settings. By default they only effect the message type and sender name.

If you want it to effect the whole message you can enable Color Whole Message in the BP_CGGControllerComponent class defaults. This will make the message look like the image below. There you can also enable or disable the time stamp of messages.