Getting field values from ProtoField

Unfortunately, that's not possible. The ProtoField only defines the format of the field; it isn't aware of packet buffers or offsets, both of which would be required to determine the field value.

On the other hand, Field extractors can pull field values from the current packet without being given buffers/offsets (the fields would already have been parsed by a dissector). However, this is only available from a tap or postdissector, and it doesn't work for Lua-defined fields (unverified).

EDIT: You might be interested in TvbRange.bitfield() and Wireshark Lua's built-in bit library, as demonstrated below.

local proto_foo = Proto('foo', 'Foo Protocol')
local f = proto_foo.fields

local LED_FLAGS = { [0] = 'off', [1] = 'on' }
f.led = ProtoField.uint8('foo.led', 'LED', base.HEX, LED_FLAGS, 0x01)

local LED_BYTE_OFFSET = 0
local LED_BIT_INDEX = 7 -- rightmost bit in MSB-0 bit numbering

function proto_foo.dissector(buf, pinfo, tree)
    -- use TvbRange.bitfield(offset, length)
    local bitval = buf(LED_BYTE_OFFSET, 1):bitfield(LED_BIT_INDEX, 1)
    print('bit', bitval, LED_FLAGS[bitval] or '?')

    -- or use the built-in "bit" library (no need to use "require")
    local num = buf(LED_BYTE_OFFSET, 1):uint()
    local bitval2 = bit.band( bit.rshift(num, 7 - LED_BIT_INDEX), 1 )
    print('bit', bitval2, LED_FLAGS[bitval2] or '?')

end