The polished nickel will have a warm gold tone depending on the manufacturer. Some deeper than others. I think I would use a color that leans toward that warm color. I personally like a dark floor, but that is personal. Some travertine will have pink undertones. If it was me I'd probably go with porcelain because I could count on the color to be the color of my sample. Some travertine samples that people have used to decide for their stone, came delivered (and sometimes installed) way different then they thought, causing a lot of disappointment and tear outs.

Can you post some pictures of your options so far? Have you looked on houzz?
Here are some Houzz pics:
<div><a href='https://www.houzz.com/photos/comfortably-modern-retreat-contemporary-bathroom-salt-lake-city-phvw-vp~104538'><img src='http://st.houzz.com/simgs/4a5125810d2e48c5_8-5656/contemporary-bathroom.jpg' border=0 width='500' height='508' /></a></div><div style='color:#444;'><small><a style='text-decoration:none;color:#444;' href='https://www.houzz.com/photos/contemporary-bathroom-ideas-phbr1-bp~t_712~s_2103'>Contemporary Bathroom</a> by <a style='text-decoration:none;color:#444;' href='http://www.houzz.com/professionals/interior-designer/c/Salt-Lake-City--UT'>Salt Lake City Interior Designers & Decorators</a> <a style='text-decoration:none;color:#444;' href='http://www.houzz.com/pro/hhdesign/handh-design'>H&H Design</a></small></div>

<div><a href='https://www.houzz.com/photos/master-bath-beach-style-bathroom-orange-county-phvw-vp~73267'><img src='http://st.houzz.com/simgs/cd7169730cabff79_8-1000/beach-style-bathroom.jpg' border=0 width='500' height='332' /></a></div><div style='color:#444;'><small><a style='text-decoration:none;color:#444;' href='https://www.houzz.com/photos/beach-style-bathroom-ideas-phbr1-bp~t_712~s_2110'>Beach Style Bathroom</a> by <a style='text-decoration:none;color:#444;' href='http://www.houzz.com/professionals/interior-designer/c/Newport-Beach--CA'>Newport Beach Interior Designers & Decorators</a> <a style='text-decoration:none;color:#444;' href='http://www.houzz.com/pro/duclett/natalie-disalvo'>Natalie DiSalvo</a></small></div>

I guess I cant copy the way I used to from houzz. So here they are, using the "houzz photo" button below and placing the "embed" info I put in above. Make sense :/

I like the second picture and think that nickel would look very nice.

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Thank you Enduring! I tried to post a pic in my original post but it said it was too long. Let me try again. Your point about the warmth in the nickel is just what I was hoping/thinking and yes, I was worried about the lack of control over the travertine. I love these concrete pavers but I don't think they're an option locally and I'm not sure if the style is right. I have no experience in that realm!

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Oh, this is cool that you can put in photos from your idea books. I didn't realize that. Anyway, that top one there they have at my local tile shop. Just the lighter color is what I was considering. And I also like this travertine that looks like it has grey and warm tones, but that's hard to know for sure unless in person and I haven't found anything like that anyway.

those are lovely examples. StonePeak is a US company.