1a) yes, I cannot enter an alpha value.
1b) Hex input would be nice for RGBA side. Maybe consider two "modes" of input so the controls on the right column would be decimal or hex.
Or, if you wanna go nuts, widen the form a little and add hex text fields next to the decimal ones to edit/see both at the same time
2) Yes. Considering the range of potential target values (percent, hex, etc) I would suggest for input an integer range of 0-100 as this is ultimately a percent that gets converted downstream to the desired target form e.g. 0.5, 50%, or 0x80.
4) OK, next one then. Having the correct value in code is more important.