I have a vague memory of the Doctor inputting code in one form or another to the Tardis, to control its appearance. I have recently re-watched Logopolis (online) and there doesn't appear to be any actual 'code' or language on-screen before the pyramid & Tardis graphics appear. Are there any other classic episodes where the Doctor shows on a screen the use of maths / programming to alter the Tardis? My recollection is possibly hex code on a pink or orange background.
Firstly, it's worth noting that, for 99.9% of the show's run, the TARDIS' appearance hasn't been controllable at all - its chameleon circuit is broken, locking it into its iconic "police box" form. Logopolis is one of only two episodes in which the Doctor attempts to fix the chameleon circuit - as of the reboot, he's decided he likes it as it is.
The only other episode in which the chameleon circuit is repaired is Attack of the Cybermen, where the Doctor explains that the TARDIS automatically disguises itself, and he has no control over what it chooses to disguise itself as. Indeed, the first time the TARDIS transforms, the Doctor is outside the TARDIS watching it happen:
Peri: I thought you said it was going to blend into its surroundings.
Doctor: Oh, she's probably thinking about it. Come on, let's find out where those signals are coming from.
[the TARDIS changes into a highly ornamented dresser]
Peri: Oh, neat, Doctor. Very neat.
Peri: Very neat. I mean, there's nothing at all incongruous about that.
Doctor: Well, she hasn't done it for a long time. She's out of practice.
(from the transcript here)
So I'm afraid your vague memory is incorrect: the Doctor cannot control the TARDIS' external appearance by any means.