Simple Javascript CAPTCHA Validation

<form id="contactForm1" onsubmit="return validateForm()" action="#" method="post">

<input type="text" name="name" required>

<input type="text" name="email" required>

<input type="text" name="phone" required>

<label>Captcha <span style="color:#F00;font-size:20px">*</span>:</label>
<input type="text" class="form-control" style="margin-bottom: 0px; width: 200px; height: 40px; font-size: 24px; font-weight: bolder; text-decoration: line-through; display: inline-block; background-color: rgb(178, 0, 255);" id="mainCaptcha" disabled="">
<button class="btn btn-success" type="button" style="display: inline-block;" id="refresh" onclick="Captcha();">Reset</button>
<div style="margin-top:10px; color: #D8000C; background-color: #FFBABA; border-radius: 7px;" id="log"></div>

<input style="margin-top:10px;" type="text" class="form-control" placeholder="Enter captcha" name="captcha" id="txtInput" required="">

<button class="btn btn-primary btn-lg" type="submit" name="submit">Submit Form</button>

</form>

function validateForm(){
            document.getElementById('log').innerHTML = '';
            var string1 = removeSpaces(document.getElementById('mainCaptcha').value);
            var string2 = removeSpaces(document.getElementById('txtInput').value);
            if (string1 != string2 || string2 == ""){
                Captcha();
                document.getElementById('log').innerHTML += '<span style="font-size:16px; padding: 25px;">Entered Invalid Captcha</span> ';
                return false;
            }
        }
        function Captcha(){
            var alpha = new Array('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','1','2','3','4','5','6','7','8','9','0');
            var i;
            for (i=0;i<6;i++){
                var a = alpha[Math.floor(Math.random() * alpha.length)];
                var b = alpha[Math.floor(Math.random() * alpha.length)];
                var c = alpha[Math.floor(Math.random() * alpha.length)];
                var d = alpha[Math.floor(Math.random() * alpha.length)];
                var e = alpha[Math.floor(Math.random() * alpha.length)];
                var f = alpha[Math.floor(Math.random() * alpha.length)];
                var g = alpha[Math.floor(Math.random() * alpha.length)];
            }
            var code = a + ' ' + b + ' ' + ' ' + c + ' ' + d + ' ' + e + ' '+ f + ' ' + g;
            document.getElementById("mainCaptcha").value = code
            var colors = ["#B40404", "#beb1dd", "#b200ff", "#faff00", "#0000FF", "#FE2E9A", "#FF0080", "#2EFE2E", ];
            var rand = Math.floor(Math.random() * colors.length);
            $('#mainCaptcha').css("background-color", colors[rand]);

        }
        function removeSpaces(string){
            return string.split(' ').join('');
        }