If it's an MPI, it can be done (and i've done it), i wanted to post a guide for it, but never find the time to take the pictures again (and what's a guide without pics?)
From memory:
Basically, for a seicento MPI, you'll need a screwdriver and some allen keys. Remove the cover that sits on the ignition barrel and sidelight switches (under the clock, 4 screws from below it), once you remove it, you'll see 2 allen bolts keeping the clock in place. Undo those, then find a third one above the clock (has a small plastic cap on it, you can see it from the outside through the windshield). Remove the 2 connectors to the clock and dismantle it. Find out which LEDs you need to change (the orange ones when you light the clock), go to your electronics store and get blue LEDs in the same size. Unsolder the old ones (taking note of the polarity - the polarity is always marked with a dent, a small triangle or otherwise on one side of the SMT LED). Solder the new ones in, again taking note of the polarity. Make sure you take note (or "before" pics) so you can put the speedo and fuel needles back the way they are supposed to be.
Reassemble and reconnect. The clocks will go through a calibration process (might twitch a few times) and then they'll be set and you'll be rewarded with a nice blue light.
I'll do a guide, i promise, but i can't say when i'll do it
Edit: don't know how to re-use the attachments, but here are the links:
Speedo:
https://www.fiatforum.com/attachment.php?attachmentid=42850&d=1209753271
Centre console:
https://www.fiatforum.com/attachment.php?attachmentid=42849&d=1209753271
It's not a hard job but will need precision soldering as the LEDs are surface mounted (around 1-2mm in length).