Hot tap is not very hot
Cold tap is lukewarm

Presumably some heat exchange is occurring between hot and cold pipes?

Anyone know how to fix it?

Warm cold water is because hot and cold pipes run next to each other under the floor – not easy to fix without re-routing all the pipes.

If hot isn't warm enough, turn up the thermostat on hot water tank or the boiler depending whether it's indirect or direct heating.

If you can get access to the pipes under the floor at some point, lag them with foam insulation if possible.

swap the blue bit on the cold tap for a red one, problem solved!

Heat transfer is not the reason, think about it, I presume the cold is off the mains, (very small possibility it is tank fed unless it's a shower or bidet, you will know by the pressure) how could it have enough time to heat up from the point it enters the house to the tap, initially yes but once running for a few moments no. I am presuming you live in a house, you may live in a block of flats which may well affect it.
The same would apply to the hot.
If the cold is tank fed it probably would be very slightly warmer than mains.
What heats the the hot? adjust it or is it faulty?
I notice a difference in the cold water temperature from summer to winter, my electric shower has to be set differently to accommodate the different incoming water temperature.

Cold water for baths and showers is normally off the header tank in the roof. Cold water for sinks will be off the mains. Warm cold water will cool off once you drain through all the water that's been sitting there getting warm in the house.

What type of boiler do you have
Combi
Boiler with header tank
Communal