If the text is colored using inline color codes:
Code:
-- turns "|cffff0000" into 255, 0, 0 for example
local r, g, b = GameTooltipTextLeft1:GetText():match("|cff(%x%x)(%x%x)(%x%x)")
r, g, b = tonumber(r, 16), tonumber(g, 16), tonumber(b, 16)
If it's colored using the SetTextColor API:
Code:
local r, g, b = GameTooltipTextLeft1:GetTextColor()