I'm with Minnie101 and Provencale in that with base units in the top shade you'd need a darker, contrasting shade to avoid something "almost but not quite" blending in. If you see the opposite in effect here, where the client wanted to use a very light tile with a beige unit (a very tricky combination) we spent time looking at the 2 colours next to each other but held at 90 degrees so we could see what the contrast was between the flat floor tile and the vertical panels etc. It turned out to be just enough to create a difference of tone.

i think the thinnest possible floor insulation is this one https://www.kingspan.com/gb/en-gb/products/insulation/insulation-boards/optim-r/optim-r-flooring-system U values here https://www.homebuilding.co.uk/insulating-floors/

How about vinyl sheeting? I’ve got it in my small shower room as I felt it was too small for tiles (so much cutting!). It’s warmer than tiles and is slightly non slip. Mine’s been down 6 years and has fared well. We had the wooden floorboards boarded with ply first. It comes in a huge array of patterns and colours. I think it all came to £250 including fitting. It’s nothing like the old style vinyl! I would go with a completely different style to the wood floor in the hall as it will be very difficult to match. Here’s some examples.

I am about to have electric UFH by ukwarmfloor.com - a norwegian system that has been around for ages over there. ( I do not work for them in any way!) Running costs are MASSIVELY cheaper than our versions. Over there, they instal the mats under roofs, drives and on the decks of icebreakers - they know a thing or two about snow and cold, and so I am giving it a whirl in my guest bedroom and large en suite. This is a response from the man answering all my many questions: "The system as shown would use a total of around 375Wper hour to keep all the areas heated to a constant 26C at floor level. By way of comparison the most efficient cable underfloor system would use 975W per hour to achieve this, whilst a boiler & radiator system would use 750W per hour. Comparisons in terms of BTUs can be a little confusing as most standard tables stating average BTU requirements for domestic situations are based on the heat output of traditional radiators etc. So our system only uses 375W (given as approx 1279BTUs in traditional thermal conversion tables)to do the job that a traditional boiler/radiator system would need 750W to do(given as approx 2559 BTUs). Our system is primarily designed specifically to produce the same air temperature in a room as other systems but to use less power to do so. The two basic reasons for this are: The heating element is self-regulating - only ever drawing the minimum required power at any individual point across the floor to maintain the set temperature. 2. The heating element heats the entire floor area and so effectively turns the whole floor of the the room into a radiant surface, heating the entire volume of air in the room from the bottom up. This is unlike radiators, which only heat a small surface, typically at the sides of a room, or a cable or piped underfloor system which can only heat thin strips of floor, leaving most of the floor surface unheated. Because our system heats the entire area it need only heat to 26C to produce effective air temperatures in the room. Other systems have to heat to much higher temperatures to do this and so require more energy to do so." I have spent an inordinate amount of time reading everything on their system, and I am going to use it. There is also a video of a customer installing it on youtube and the link is on their site somewhere.