Is there a design format that allows you to place a logo in Word on a coloured background, without a white box around the logo?

I was going to recommend .gif or .png, until I realized you had tried that. I’m not sure why .gif and .png don’t work, because they do for me. Are you sure transparency exists in the original .gif / .png files themselves?

Not sure about other versions, but in Word ‘07, you may also use the format picture tools and set a transparent color (note that it will take out that color everywhere in the logo).

If I’m correctly understanding your question, how about if you fill the box with the same color that’s around it?

.png should work, just make sure you are using transparency and not just white.

Easiest way would be as @Jeruba has told, make background colour of logo and background colour of page same.

There’s another way like @J0E has told, which you might be looking for. In word 2007, after you’ve insert -ed the logo (any picture format, jpg, png, etc.), click on the logo. Now, a tab called “Picture tools: Format” will appear as @dotun_o has suggested. In it, go in “Adjust” area, in it, you’ll find a feature called “Recolor”. In it, click on “Set Transaparent Color”, then the cursor changes to dropper like form. Click on the logo background (the white space) that you want to make transparent, and lo, it’s done!

As @J0E said, the PNG files will do the trick but the background has to be transparent. What software are you using to create the logo?